Abstract: WB2 well block of Weibei oilfield adopts the method of direct injection and production, but the development effect is not ideal. Through laboratory experiment of physical models of Chang 3 ultra–low permeability reservoirs in Weibei oilfield, in combination with nuclear magnetic resonance data, the microscopic displacement mechanism of water flooding, CO2 flooding and the manner of water injection to gas injection flooding was studied.The experimental results indicate that the water flooding efficiency is the lowest(37%), the CO2 flooding efficiency is higher(54.3%), and the oil efficiency of water flooding to gas flooding is the highest(about 55.7%). By use of the nuclear magnetic resonance(NMR) to measure different displacement models of the cores, it is found that the residual oil distribution is directly used in medium and small pores in water flooding, while gas flooding is directly used in crude oil of medium and large pores, and the manner of water flooding to gas flooding can be expanded into a larger cope, it can be further used in the crude oil of medium pores. It is feasible of CO2 injection after water injection to enhance the displacement efficiency for Chang 3 reservoirs in Weibei oilfield.