Abstract: To improve the job quality of well cementing, the oil base mud and oil films adhered on the surface of the borehole to be cemented have to be removed and the mixture of oil base drilling fluid and spacers in the borehole should be changed to water wetting. Two wettability reversal agents, PC-W31S and PC-W25L, have been developed to perform the tusks mentioned above. PC-W31S is added at a concentration between 0.5% and 1.0% (percent mass of water) and PC-W25L between 5% and 10%. Study of the working mechanisms of the two agents has been conducted, focusing on the effects of the wettability reversal agents on the water wetting capacity of spacers used. In the study, PC-W31S and PC-W25L were compounded to make the best use of the two. Evaluation tests of the two agents showed that when the volumetric fraction of a spacer treated with PC-W31S and PC-W25L (as the main additives) in a mixture of the spacer and oil base mud is less than 50%, the mixture can be completely turned from oil wetting (oil as the continuous phase) to water wetting (water as the continuous phase). Using the spacer to flush a rotor, more than 80% of the oil base mud adhered on the surface of the rotor can be flushed away and the flushed surface becomes water wet. The spacer is rheologically compatible with oil base drilling fluids and cement slurries, and the effects of the spacer on the thickening time and compressive strength of cement slurry are controllable. The two agents have been successfully applied on the well LD8-1-7 and won a bidding in a well cementing project in southeast Asia.