A method is provided to heat a target device of a computer system by a heater equipped therein. The target device is heated to reach a target temperature through multiple temperature intervals. The method senses the current device temperature of the target device to compare with the preset interval temperature of the temperature interval and obtain a phase difference for adjusting a heating power. When heating the target device by the heating power, the current preset interval temperature of the current temperature interval is increased progressively by a preset temperature rising value to obtain the next preset interval temperature of the next temperature interval. Repeat the foregoing steps in every temperature interval until the preset interval temperature or the device temperature reaching the target temperature.