An automatic gear-shifting bicycle includes: a bicycle body provided with two cranks, two pedals and a derailleur, a power supply module, a microcomputer, a gear shifting control driver, and a pedal position sensor module installed in the bicycle body corresponding to the cranks and electrically coupled to the microcomputer for enabling the microcomputer to determine the angular position and forward/backward pedaling of the cranks. The microcomputer calculates the optimal shift timing subject to the time point the crank to be moved over the pedaling dead point in the next time, the pedaling speed to be below a predetermined speed value or the cranks are been pedaling backwardly.