Fc fc = new Fc() {}; var temp = Builders<MModel>.Filter.Where(m=>m.id== "882d4d22-ff70-41f9-beae-ccc358fd9d7d"); var filter = Builders<Model>.Filter.Eq("_id", id); var document = collection.Find(filter).FirstAsync().Result; var update = Builders<Model>.Update.Push("contents.phon", fc); var result = collection.UpdateOneAsync(filter, update).Result;