It is apt to produce frequency modulation signals of FSK communication system. The system equipment is simple. It owns an outstading characteristic of anti-interference. The system based on MCS-51 Microcontroller has simple circuit, structure stable performance and low research cost. This article describes the implementation based on AT89S52 FSK modem design. The system consists of FSK modulator, demodulator, serial communication circuit, D/Aconverter circuit, low pass filter, and waveformshaping circuit is completed in the Keil software design, hardware simu- lation completed in Proteus and Multisim. The system is finally tested on a general board and its design feasibility is also verified.