Abstract: In the mining process in No. 1121 fully mechanized caving face, especially during initial mining period, CO over limit in top corner had happened many times. To explore reasons of CO over limit, which coal spontaneous combustion in the goaf or shot-firing smoke in roadway head and other causes，We conducted sampling tests on gas in top corner for two months，increasely monitored and took the corresponding measures. The results showed that to decrease air-leak-age in the goaf can play a role in the prevention of coal oxidation，but the effect was not satisfied. In connection with CO maybe coming from coal spontaneous combustion in the goaf and shot-firing smoke in roadway head，we increased the air volume by air pipe in top corner in working face, and then CO over limit were effective controlled.