Abstract: In order to solve the problem of performance deterioration of cement slurry at high temperature,a thermo-stable cement slurry system was developed with DRF-120L as the main additive in the cement slurry based on the thermostable fluid loss additive DRF-120L,by means of optimizing other compatible additives.Evaluations on performance of the cement slurry were carried out based on laboratory analysis.The results showed that the cement slurry could be used at the temperature up to 200℃;it has good salt tolerance property,and even used in saturated brine,it could meet the high-temperature cementing requirements in deep wells;its fluid loss could be controlled under 100 mL,thickening time could be easily adjusted,compressive strength of set cement was high.Pilot tests of this cementing system were conducted in seven wells in Tarimu,Liaohe and Huabei Oilfields,cementing quality of these wells was qualified.This indicated that comprehensive performance of this kind of thermostable cement slurry system could meet the technical requirements for high temperature and deep well cementing,the problem of performance deterioration of cement slurry at high temperature had been solved.