Miscible，near-miscible and immiscible state coexist during CO2 flooding in low permeability reservoirs due to the large injection-production pressure difference between injection well and production well. It is not suitable to utilize MMP to evaluate CO2 flooding effect. So traditional capillary number theory was modified for the low permeability oil reser? voirs considering diffusion effect，swelling，viscosity reduction，interfacial tension reduction，variation on viscosity ratio， density ratio and wettability. In combination of PVT phase behavior and displacement experiments of CO2 flooding in low permeability long cores before and after CO2 solution，the modified capillary number theory was utilized to optimize the in? jection-production parameters of CO2 flooding in Yaoyingtai low permeability reservoirs. The results show that the critical capillary number exists and ranges from 4.5×10-7 to 6.0×10-7，and the optimum injection rate is 0.4 mL/min and the average injection pressure is 28.84-33.43 MPa for CO2 flooding. The optimized injection-production parameters have been applied in CO2 flooding pilot test in Yaoxi block in Yaoyingtai oilfield since April 2011. Till to March 2015，incremental CO2 injec? tion has achieved 16.7×104 m3，incremental oil production has surpassed 19 253 tons，which indicates the good develop? ment effect and economic benefit of CO2 flooding.