Abstract: We address the problem of not being able to detect the conductivity change of mixed liquid (oil-based drilling fluid and flushing fluid) in oil-based drilling fluid. Flushed by oil-in-water based flushing fluid with the wetting tester, the proper surfactant and solvent were optimized and compounded, and a nitrogen-containing polymer was introduced to develop the PC–W31L flushing fluid for oil-based drilling fluid. The flushing mechanism of this fluid on oil-based drilling fluid was analyzed and its performance was evaluated. The results showed that the flushing effect of PC–W31L flushing liquid on oil-based drilling fluid was optimal, the flushing interface was in water wet state, which can completely transform oil-based drilling fluid from the oil continuous phase to the water continuous phase. The rheology compatibility of this fluid is better with oil-based drilling fluid and cement slurry respectively, and its influences on the thickening time and strength of cement slurry are all within the controllable range. The results of performance evaluation indicated that PC–W31L flushing liquid not only had a good flushing effect on oil-based drilling fluid, but also allowed the wetting tester to detect its water wetting capacity.