In order to study the effect of formation water salinity on the oil displacement efficiency of CO2 flooding，using Berea cores with similar porosity and permeability parameters，the core displacement method was used to test the CO2 displacement efficiency of saturated 0，6778 and 15000 mg/L water，the changes of porosity and permeability，the radius changes of pore and pore throat before and after CO2 flooding were analyzed by CT scanning. The influence mechanism of water salinity on CO2 displacement efficiency was analysed. The results showed that with the increase of the salinity of saturated water in the cores，the precipitation produced in the process of CO2 flooding increased，and the pores and throats were blocked seriously. The radius of pore and throat decreased，and the porosity and permeability decreased，resulting in the decrease of the minimum miscible pressure and the CO2 displacement efficiency. It shows that the salinity of formation water has an effect on the oil displacement efficiency of CO2 flooding.