The well pattern infilling is one of the important measures to improve the development performance of the complicated faultblock reservoirs at high water cut stage.And，the determination of reasonable injection-production well spacing is a critical factor affecting the development performance of well pattern infilling.Taking the well group of one injection well and two production wells for example，a new method，based on the non-steady oil-water two-phase flow theory to calculate reasonable well spacing，is constructed to achieve the equilibrium displacement for the complicated fault-block reservoirs at high water cut stage with the influence of such typical factors as reservoir parameters，remaining oil distribution and formation dip.And，it’s verified by the reservoir numerical simulation.The results indicate that this method can achieve the equilibrium displacement and improve the developing performance greatly.The research is of important guiding significance to the determination of reasonable well spacing for the complicated fault-block reservoirs at high water cut stage.