In the process of the fourth hole (WFSD-4) of Wenchuan earthquake scientific drilling project, the drilling fluid was contaminated by CO2 seriously and the drilling fluid performance was deteriorated with sticking & cutting increasing and poor liquidity, by which normal drilling was affected. Through the analysis on the influence factors of CO2 on drilling fluid and according to the mechanism and characteristics of drilling fluid contamination by CO2, the contamination rules, performance characteristics and corresponding treatment measures are summed up based on the laboratory experiments and field application cases: with pH value of drilling fluid keeping, acid radical ion etc. in drilling fluid can be removed by lime.