转载：http://www.cnblogs.com/atsats/p/6607886.html
如果未接显示设备，使用VNC登录后，显示分辨率很小，应该是480p，导致使用很不方便。
这里通过修改/boot/config.txt文件，配置不接显示器时也能按照设定的分辨率显示桌面。具体操作如下：
1.打开config.txt文件
方法一：用SD卡读卡器，在windows环境下，boot文件夹中打开config.txt文件
方法二：进入Raspberry系统后，
a.使用vi编辑器：sudo vi /boot/config.txt
b.使用图形界面的Editor：sudo leafpad /boot/config.txt (很适合不熟悉vi的同学)
2. 修改config.txt文件中如下参数值
去掉文件中的第一个“#”，然后更新数值
hdmi_force_hotplug=1 #即便没有检测到HDMI显示器也要使用HDMI模式
hdmi_drive=2 #HDMI模式 (如果支持并已启用将有声音输出)
hdmi_group=2 #使用DMT格式
hdmi_mode=16 #DMT格式下的屏幕分辨率，这里是1024x768
config.txt文件参数详细说明参考此文章：
http://shumeipai.nxez.com/2015/11/23/raspberry-pi-configuration-file-config-txt-nstructions.html
3. 保存config.txt文件后，不插HDMI电缆，重启树莓派验证效果。
祝您成功！
注：具体分辨率配置说明如下
hdmi_mode 设置屏幕分辨率
当hdmi_group=1 (CEA)时,下列值有效hdmi_mode=1 VGA
hdmi_mode=2 480p 60Hz
hdmi_mode=3 480p 60Hz H
hdmi_mode=4 720p 60Hz
hdmi_mode=5 1080i 60Hz
hdmi_mode=6 480i 60Hz
hdmi_mode=7 480i 60Hz H
hdmi_mode=8 240p 60Hz
hdmi_mode=9 240p 60Hz H
hdmi_mode=10 480i 60Hz 4x
hdmi_mode=11 480i 60Hz 4x H
hdmi_mode=12 240p 60Hz 4x
hdmi_mode=13 240p 60Hz 4x H
hdmi_mode=14 480p 60Hz 2x
hdmi_mode=15 480p 60Hz 2x H
hdmi_mode=16 1080p 60Hz
hdmi_mode=17 576p 50Hz
hdmi_mode=18 576p 50Hz H
hdmi_mode=19 720p 50Hz
hdmi_mode=20 1080i 50Hz
hdmi_mode=21 576i 50Hz
hdmi_mode=22 576i 50Hz H
hdmi_mode=23 288p 50Hz
hdmi_mode=24 288p 50Hz H
hdmi_mode=25 576i 50Hz 4x
hdmi_mode=26 576i 50Hz 4x H
hdmi_mode=27 288p 50Hz 4x
hdmi_mode=28 288p 50Hz 4x H
hdmi_mode=29 576p 50Hz 2x
hdmi_mode=30 576p 50Hz 2x H
hdmi_mode=31 1080p 50Hz
hdmi_mode=32 1080p 24Hz
hdmi_mode=33 1080p 25Hz
hdmi_mode=34 1080p 30Hz
hdmi_mode=35 480p 60Hz 4x
hdmi_mode=36 480p 60Hz 4xH
hdmi_mode=37 576p 50Hz 4x
hdmi_mode=38 576p 50Hz 4x H
hdmi_mode=39 1080i 50Hz reduced blanking
hdmi_mode=40 1080i 100Hz
hdmi_mode=41 720p 100Hz
hdmi_mode=42 576p 100Hz
hdmi_mode=43 576p 100Hz H
hdmi_mode=44 576i 100Hz
hdmi_mode=45 576i 100Hz H
hdmi_mode=46 1080i 120Hz
hdmi_mode=47 720p 120Hz
hdmi_mode=48 480p 120Hz
hdmi_mode=49 480p 120Hz H
hdmi_mode=50 480i 120Hz
hdmi_mode=51 480i 120Hz H
hdmi_mode=52 576p 200Hz
hdmi_mode=53 576p 200Hz H
hdmi_mode=54 576i 200Hz
hdmi_mode=55 576i 200Hz H
hdmi_mode=56 480p 240Hz
hdmi_mode=57 480p 240Hz H
hdmi_mode=58 480i 240Hz
hdmi_mode=59 480i 240Hz H
H表示16:9比例(正常是4:3).
2x表示双倍像素(即更高的像素时脉, 每个像素重复两次)
4x表示四倍像素(即更高的像素时脉, 每个像素重复四次)
当hdmi_group=2 (DMT)时,下列值有效警告: 像素时脉是有限制的, 最高支持的模式是1920x1200 @60Hz with reduced blanking.
hdmi_mode=1 640x350 85Hz
hdmi_mode=2 640x400 85Hz
hdmi_mode=3 720x400 85Hz
hdmi_mode=4 640x480 60Hz
hdmi_mode=5 640x480 72Hz
hdmi_mode=6 640x480 75Hz
hdmi_mode=7 640x480 85Hz
hdmi_mode=8 800x600 56Hz
hdmi_mode=9 800x600 60Hz
hdmi_mode=10 800x600 72Hz
hdmi_mode=11 800x600 75Hz
hdmi_mode=12 800x600 85Hz
hdmi_mode=13 800x600 120Hz
hdmi_mode=14 848x480 60Hz
hdmi_mode=15 1024x768 43Hz DO NOT USE
hdmi_mode=16 1024x768 60Hz
hdmi_mode=17 1024x768 70Hz
hdmi_mode=18 1024x768 75Hz
hdmi_mode=19 1024x768 85Hz
hdmi_mode=20 1024x768 120Hz
hdmi_mode=21 1152x864 75Hz
hdmi_mode=22 1280x768 reduced blanking
hdmi_mode=23 1280x768 60Hz
hdmi_mode=24 1280x768 75Hz
hdmi_mode=25 1280x768 85Hz
hdmi_mode=26 1280x768 120Hz reduced blanking
hdmi_mode=27 1280x800 reduced blanking
hdmi_mode=28 1280x800 60Hz
hdmi_mode=29 1280x800 75Hz
hdmi_mode=30 1280x800 85Hz
hdmi_mode=31 1280x800 120Hz reduced blanking
hdmi_mode=32 1280x960 60Hz
hdmi_mode=33 1280x960 85Hz
hdmi_mode=34 1280x960 120Hz reduced blanking
hdmi_mode=35 1280x1024 60Hz
hdmi_mode=36 1280x1024 75Hz
hdmi_mode=37 1280x1024 85Hz
hdmi_mode=38 1280x1024 120Hz reduced blanking
hdmi_mode=39 1360x768 60Hz
hdmi_mode=40 1360x768 120Hz reduced blanking
hdmi_mode=41 1400x1050 reduced blanking
hdmi_mode=42 1400x1050 60Hz
hdmi_mode=43 1400x1050 75Hz
hdmi_mode=44 1400x1050 85Hz
hdmi_mode=45 1400x1050 120Hz reduced blanking
hdmi_mode=46 1440x900 reduced blanking
hdmi_mode=47 1440x900 60Hz
hdmi_mode=48 1440x900 75Hz
hdmi_mode=49 1440x900 85Hz
hdmi_mode=50 1440x900 120Hz reduced blanking
hdmi_mode=51 1600x1200 60Hz
hdmi_mode=52 1600x1200 65Hz
hdmi_mode=53 1600x1200 70Hz
hdmi_mode=54 1600x1200 75Hz
hdmi_mode=55 1600x1200 85Hz
hdmi_mode=56 1600x1200 120Hz reduced blanking
hdmi_mode=57 1680x1050 reduced blanking
hdmi_mode=58 1680x1050 60Hz
hdmi_mode=59 1680x1050 75Hz
hdmi_mode=60 1680x1050 85Hz
hdmi_mode=61 1680x1050 120Hz reduced blanking
hdmi_mode=62 1792x1344 60Hz
hdmi_mode=63 1792x1344 75Hz
hdmi_mode=64 1792x1344 120Hz reduced blanking
hdmi_mode=65 1856x1392 60Hz
hdmi_mode=66 1856x1392 75Hz
hdmi_mode=67 1856x1392 120Hz reduced blanking
hdmi_mode=68 1920x1200 reduced blanking
hdmi_mode=69 1920x1200 60Hz
hdmi_mode=70 1920x1200 75Hz
hdmi_mode=71 1920x1200 85Hz
hdmi_mode=72 1920x1200 120Hz reduced blanking
hdmi_mode=73 1920x1440 60Hz
hdmi_mode=74 1920x1440 75Hz
hdmi_mode=75 1920x1440 120Hz reduced blanking
hdmi_mode=76 2560x1600 reduced blanking
hdmi_mode=77 2560x1600 60Hz
hdmi_mode=78 2560x1600 75Hz
hdmi_mode=79 2560x1600 85Hz
hdmi_mode=80 2560x1600 120Hz reduced blanking
hdmi_mode=81 1366x768 60Hz
hdmi_mode=82 1080p 60Hz
hdmi_mode=83 1600x900 reduced blanking
hdmi_mode=84 2048x1152 reduced blanking
hdmi_mode=85 720p 60Hz
hdmi_mode=86 1366x768 reduced blanking
笔者使用的镜像2018-11-13-raspbian-stretch.img