The present invention relates to an energy-saving system for three-phase induction motors and is based on the principle of variable voltage control at constant speed. It is composed of a microprocessor, exampling circuit, sensing circuit, acquiring circuit, firing circuit, and AC to AC converter. The system is to automatically adjust the voltage to the induction motor with the variation in the motor load, in order to obtain high operating power factor and efficiency. The system will result in considerable energy-savings when a three-phase induction motor runs under constant light-load or variable-load with low duty ratio.