Displacement efficiency and interface stability are critical to the success of offshore extended reach well cementing. According to the theory of computational fluid dynamics and the basic cementing data of extended reach well in South China Sea, the cement slurry rheological property and the influence of different borehole conditions on cementing displacement are obtained by numerical simulation. The calculations demonstrate that displacement efficiency increases with the increase of cement slurry consistency index and decreases with the increase of cement slurry flow index; well-cementing displacement efficiency first increases and then decreases and finally tends to be stable as the yield stress increases; the flow index of cement slurry should not be too high when cementing, and the value of n should be controlled within 0.6; the consistency index should be greater than 1.5 Pa·sⁿ, but not exceed 3 Pa·sⁿ; yield stress in the range of 10~18 Pa is advantageous to replace drilling fluid with the cement slurry. It is suggested that the eccentricity should be controlled within 0.5 as eccentricity has a great influence on the stability of interface; the stability of interface is obviously improved by using casing centralizer, and the optimized best rigid centralizer swirl angle is 60°; casing buckling, causing significant reduction of displacement efficiency in the annular interface, has a great influence on the stability of interface.