For MO-PA (master-oscillator pouver amplifier) excimer laser dual-chamber discharge synchronous system, a square wave signal generator based on PIC16F873A microcontroller and DDS (direct digital synthesizer, direct digital frequency synthesizer) chip AD9833 is designed. Comparing to advantages and disadvantages of the traditional simple signal generator and considering application environment of laser operating, the hardware structure of the system and the software process to achieve were discussed. Through experiments and analysis, the results show that the output signal circuit can generate high quality square wave ranging within 1～4 kHz and the frequency error is less than 0.05 Hz.