Solution:
When playing the sources in the online film, click the menu key on the Mi remote control and select the definition. When playing the sources in the online film next time, the definition will be played according to the current setting.
For example, when playing a video, the definition of the video will be changed to super clear. Then playing the next video with super clear definition. If the video be played again without an HD version, it will be played at maximum resolution (HD or starand Definition).