Abstract: Checking poison gas is a import tache in fathering the polluted surrounding,in special surrounding such as poison gas and poison gas leaking slowly.The reliability and stability of poison gas checked is always the keystone and difficulty of the gas checked instrument.The text adopted the zero excursion technology of “no zero when working,zero when no working” which developed inspect/control equipment of poison gas,via alternating work of two sensor and enter route controlled method which settled the inspect/control equipment's problem of misinformation,fail to report in complex surrounding.