A power-assisted bicycle includes a bicycle body with a power assistance provider and a derailleur, a microcomputer, a gear-shift control driver, and sensors. When the microcomputer decides to perform gear shifting, it controls the power assistance provider to provide an assistant power output and the gear-shift control driver to drive the derailleur to shift gears properly subject to a before-shifting time period, an in-shifting time period and an after-shifting time period. Further, the microcomputer controls the power assistance provider to change the current assistant power to a predetermined low assistant power during the before-shifting time period, or to maintain the predetermined low assistant power during the in-shifting time period, or to change the assistant power output to a level higher than the current assistant power and then to return to the said current assistant power after the end of the after-shifting time period.