At present, water sealing fire resistant explosion venting devices with artificial filling water type are often used for the transmission of the low concentration of coal mine gas pipeline, which have stronger artificial dependence, poorer real-time and lower efficientcy. Based on PLC S7-200, the water sealing fire resistant explosion venting devices with automatic control function of constant water level, timely detection of gas pipeline pressure and gas concentration were designed. The principle of water sealing fire resistan explosion venting device was elaborated; according to the detailed design technology indicators, the hardware and the selection of PLC device were selected, and the configuration of PLC resource was configured， and the main software of the device was designed. The design of the device is better to realize the function of constant water level control, and real-time detection and display of the gas pressure and gas concentration in the tank body.