The relationship between the characteristics of change of CO₂ in fault product gas in Houhaoyao,Huailai county in 1998 and influence factors,such as geotemperature and rainfall is analysed.The observation values of CO₂ in fault product gas and geotemperature change generally in synchronism.The reason of negative correlation between change of CO₂ and geotemperature is discussed.The negative correlation from March to early in June,1998 resulted from recovery process of CO₂ observation values from high to low after the Zhangbei-Shangyi M_S6.2 earthquake.The negative correlation from the middle of June to the middle of Sept.,1998 resulted from excessive rainfall during this period.The anomalous characteristics of CO₂ before and after the Xilinhaote M_S5.2 earthquake,Inner Mongolia on January 29,1999 and the Zhangbei M_S5.6 strong aftershock on March 11,1999 are introduced.