Strong base ASP flooding technology can improve swept volume, displacement efficiency and oil recovery greatly. It shows encouring prospects for sustainable development of the maturing oilfield. So In this paper, with the help of data from coring wells and newly-drilled wells after ASP flooding, we has strengthened understanding of the reservoir.By using numerical simulation, carrying out lab core displacement test, microscopic oil displacement test, and using techniques such as nuclear magnetic resonance, electron microscopic detection and electrical logging curves of newly-drilled wells and so on, we have found out petrophysical changes in reservoirs after ASP, evaluated oil displacement efficiency of different locations, and systematically analyzed influcing factors on occurrence of displacement efficiency differences. The study results show that the oil displacing efficiency increased by 14.7% after strong alkali ASP flooding, and the thickness of heavy-water-flooded layer increasedby 26.0%. Chemical reactions and ion exchanges between ASP fluid, rock and clay minerals lead to changes of reservoir physical properties: Shale content increased; median grain diameter decreased; Permeability of intervals lowered and the wettability is converted to hydrophilicity. petrophysical changes of the reservoir is the main factor influencing oil displacement efficiency.The research result provides a guidance to explore potentialities of ASP flooding during the commercial popularization process and it is an important reference for further studies on enhancing oil recovery techniques after ASP flooding.