In a torque control system and method for power impact torque tool, first establish a curve showing the relationship between the high/low working voltages of the tool in normal operation and the high/low torque values that are correspondingly output by the tool at the high/low working voltages. Input a target torque value that falls between the highest and the lowest torque value on the relationship curve to obtain a working voltage corresponding to the target torque value for performing a tightening operation. During the tightening process, a microprocessor of a torque control device receives and uses sensing signals instantly and continuously sent by a torque sensing device and changes in voltage, current and motor temperature continuously detected by a voltage/current sensing element and a temperature sensing element to stably control, via a voltage control module, the working voltage within a preset allowable range of variation for tightening torque control.