A shift control system configured to work with a power-assisted bicycle that has an assisting-power supplying system and a derailleur, the shift control system comprising: a microcomputer storing therein a plurality of shifting condition tables (19a,19b) and being electrically connected to the assisting-power supplying system, so as to control the assisting-power supplying system to provide at least two assisting modes that have different assisting-power output ratios; a shifting driver being electrically connected to the microcomputer and controlled by the microcomputer to conduct automatic shifting; and at least one sensor being deposited at a predetermined location on the power-assisted bicycle and electrically connected to the microcomputer, for sensing at least one of a wheel speed, a crank speed, a pedaling force and a road gradient so as to generate at least one sensing signal that is transmitted to the microcomputer, so that the microcomputer produces at least one sensing result according to the at least one sensing signal, wherein the microcomputer selects one of the shifting condition tables (19a,9b) in accordance with the current assisting mode of the assisting-power supplying system for comparing the at least one sensing result, thereby determining whether Upshifting or downshifting is to be conducted, and then controls the shifting driver to send a shift control signal to the derailleur for making the derailleur conduct upshifting or downshifting: characterized in that the shift control system further comprises a gear-simplifying logic, which is stored in the microcomputer and serves to simplify a number of gear positions into a smaller number of the gear positions, wherein after determining whether upshifting or downshifting is to be conducted, the microcomputer additionally uses a predetermined condition to determine whether shifting is to be conducted according to the gear-simplifying logic, wherein the predetermined condition referred by the microcomputer is the current assisting mode. a microcomputer storing therein a plurality of shifting condition tables (19a,19b) and being electrically connected to the assisting-power supplying system, so as to control the assisting-power supplying system to provide at least two assisting modes that have different assisting-power output ratios; a shifting driver being electrically connected to the microcomputer and controlled by the microcomputer to conduct automatic shifting; and at least one sensor being deposited at a predetermined location on the power-assisted bicycle and electrically connected to the microcomputer, for sensing at least one of a wheel speed, a crank speed, a pedaling force and a road gradient so as to generate at least one sensing signal that is transmitted to the microcomputer, so that the microcomputer produces at least one sensing result according to the at least one sensing signal, wherein the microcomputer selects one of the shifting condition tables (19a,9b) in accordance with the current assisting mode of the assisting-power supplying system for comparing the at least one sensing result, thereby determining whether Upshifting or downshifting is to be conducted, and then controls the shifting driver to send a shift control signal to the derailleur for making the derailleur conduct upshifting or downshifting: characterized in that the shift control system further comprises a gear-simplifying logic, which is stored in the microcomputer and serves to simplify a number of gear positions into a smaller number of the gear positions, wherein after determining whether upshifting or downshifting is to be conducted, the microcomputer additionally uses a predetermined condition to determine whether shifting is to be conducted according to the gear-simplifying logic, wherein the predetermined condition referred by the microcomputer is the current assisting mode. characterized in that the shift control system further comprises a gear-simplifying logic, which is stored in the microcomputer and serves to simplify a number of gear positions into a smaller number of the gear positions, wherein after determining whether upshifting or downshifting is to be conducted, the microcomputer additionally uses a predetermined condition to determine whether shifting is to be conducted according to the gear-simplifying logic, wherein the predetermined condition referred by the microcomputer is the current assisting mode.