A PWM step wave and sine wave driving device is provided. The driving device includes an ADC unit, an input processing unit, a first pulse width calculation unit, a register unit, a first output unit, a zero point detecting unit, a second pulse width calculation unit, a second output unit, and a multiplexer output unit. The driving device receives input signals, and output desired step wave or sine wave driving signals for driving an external electric device. The driving device detects a point of zero voltage of the system power supply by the zero point detecting unit. When the system power supply is abnormal, the first output unit outputs predetermined PWM step wave output signals, or the second output unit outputs predetermined PWM sine wave output signals. In such a way, the external electric device can be maintained for regular operation, so that the malfunction or breakdown thereof is prevented.