A method and a system for regulating power in a portable audio/video playback device enabling the portable audio/video playback device to completely play an audio/video multimedia file or data without sufficient power or external power. A processing unit is employed for detecting a remaining power of the portable audio/video playback device and for computing an average power consumption rate and a remaining playback time of the audio/video multimedia file or data to thereby appropriately reduce an output level of the video or audio output signals. Accordingly, power consumption of the portable audio/video playback device is minimized so that a complete playback of the audio/video multimedia file or data can be performed by the portable audio/video playback device before the power is exhausted.