In view of the characteristics of poor CO2 flooding effect and serious gas channeling phenomenon in low permeability and ultra-low permeability reservoirs，water alternate gas（WAG）improving CO2 flooding effect was investigated. Taking reservoir physical simulation as the main research method，the impacts of low permeability and ultra-low permeability core and heterogeneity on the WAG flooding effect was studied. Selecting natural outcrops and artificial heterogeneous cores，and the injection method， injection speed，injection parameters and injection timing were researched and optimized for WAG flooding. The results showed that WAG flooding could achieve good mobility control and prolong CO2 channeling time for low permeability and ultra-low permeability homogeneous cores of 0.5×10-3 μm2，1×10-3 μm2 and 5×10-3 μm2，and the lower the permeability，the later the gas channeling time. As for the heterogeneous cores with the permeability differential of 5（1×10-3/5×10-3 μm2 ），10（1×10-3/10×10-3 μm2） and 50（1×10-3/50×10-3 μm2 ），the smaller the permeability differential ，the higher the gas-water ratio，the higher the enhanced oil recovery. When The permeability differential was 10，the gas channeling time was obviously advanced，especially when the permeability differential was greater than 50，the WAG slug could not start the remaining oil in the low-permeability matrix，and only a small part of the crude oil could be displaced in gas injection stage. WAG in use within limit adaptation could significantly reduce the CO2 mobility and prolong CO2 channeling time，start the residual oil in matrix and enhanced the residual oil recovery.