Abstract: Based on the CPLD, this paper designed a scheme of serial data communication of the ultrasonic alarming system for gas SF6. Using the language VC and VHDL to realize the communication program of the up-device and down-device respectively. The practice experiment proved there are merits of high baud rate and actual transmittion about the serial communication by CPLD. The serial data communication is adapt for the detecting and alarming system with weak signal.