Abstract: Wells drilled with oil base drilling fluid always have poor cementing job quality because of the contamination to the cement slurry by the oil base drilling fluid. Studies on the cement slurries that are resistant to contamination have been conducted using experimental method and theoretical analysis. In laboratory experiments with artificial cores, the effects of oil base drilling fluid on the cementing job quality, especially the effects of oil base drilling fluid to the bonding of the cement sheath and the borehole wall, have been proved. Anti-P (main component being alkylphenolpolyethoxylate), an anti-contamination agent for cement slurry was developed by studying the change of the properties of base cement slurry before and after being contaminated with oil base drilling fluid. Laboratory studies showed that cement slurry treated with Anti-P can stand the contamination by oil base drilling fluid of less than 10%, the properties of the cement slurry was not affected, and the cementing job quality satisfied the requirements. The application of Anti-P in several wells has gotten good results. This technology has filled the gap of anti-contamination cement slurry and is well worth further study and application.