2. When iTunes recognizes the mobile phone, click the music in the function list database on the left side of the application.

3. Select the option to add a folder to the library, select a folder, and then click OK. ITunes will import all music files in this folder into the iTunes library.

4. After importing music files into iTunes library, you can synchronize music files.

5. Select the iPhone in the iTunes device, click Music, check the music, and then select the "Entire Music Library" option.

6. After setting, click the Apply button at the bottom to synchronize all the added music to the iPhone. If only some music in the music library is synchronized, select the "Selected Playlist, Artist and Style" option, and then select the playlist, artist and style to synchronize.