A gear-shifting system for a power-assisted bicycle, wherein the gear-shifting system operates according to a pedal position of the power-assisted bicycle that has a motor and a gear shifter, the gear-shifting system comprising: a power supply module; a microcomputer, electrically connected to the power supply module and the motor, for controlling an assisting power output by the motor; a gear-shifting driver, electrically connected to the microcomputer and physically coupled to the gear shifter, so as to be controlled by the microcomputer to drive the gear shifter to perform a gear-shifting operation; and a pedal position sensing module, deposited on the power-assisted bicycle, aligned with at least one crank of the power-assisted bicycle, and electrically connected to the microcomputer, so as to inform the microcomputer of an angular position of the at least one crank; the microcomputer having a gear-shifting operation logic, and being configured to execute the gear-shifting operation logic when determining by itself that the gear-shifting operation shall be performed or when receiving a gear-shifting command from a cyclist; characterized by: the microcomputer executing the gear-shifting operation logic by performing steps of: when the angular position of the at least one crank passes through a first angular position, reducing the assisting power output by the motor to a first power value; when the angular position of the at least one crank leaves the first angular position and passes through a dead point (Pd), minimizing the assisting power output by the motor; controlling the gear-shifting driver to drive the gear shifter to perform the gear-shifting operation; after the gear-shifting operation, returning the assisting power output by the motor to a level of the assisting power before the step. a power supply module; a microcomputer, electrically connected to the power supply module and the motor, for controlling an assisting power output by the motor; a gear-shifting driver, electrically connected to the microcomputer and physically coupled to the gear shifter, so as to be controlled by the microcomputer to drive the gear shifter to perform a gear-shifting operation; and a pedal position sensing module, deposited on the power-assisted bicycle, aligned with at least one crank of the power-assisted bicycle, and electrically connected to the microcomputer, so as to inform the microcomputer of an angular position of the at least one crank; the microcomputer having a gear-shifting operation logic, and being configured to execute the gear-shifting operation logic when determining by itself that the gear-shifting operation shall be performed or when receiving a gear-shifting command from a cyclist; characterized by: the microcomputer executing the gear-shifting operation logic by performing steps of: when the angular position of the at least one crank passes through a first angular position, reducing the assisting power output by the motor to a first power value; when the angular position of the at least one crank leaves the first angular position and passes through a dead point (Pd), minimizing the assisting power output by the motor; controlling the gear-shifting driver to drive the gear shifter to perform the gear-shifting operation; after the gear-shifting operation, returning the assisting power output by the motor to a level of the assisting power before the step. the microcomputer executing the gear-shifting operation logic by performing steps of: when the angular position of the at least one crank passes through a first angular position, reducing the assisting power output by the motor to a first power value; when the angular position of the at least one crank leaves the first angular position and passes through a dead point (Pd), minimizing the assisting power output by the motor; controlling the gear-shifting driver to drive the gear shifter to perform the gear-shifting operation; after the gear-shifting operation, returning the assisting power output by the motor to a level of the assisting power before the step.