In view of the practical problems of low permeability and high salinity reservoirs in Yanchang oilfield，such as poor salt tolerance of conventional foam injection and bad oil displacement efficiency，a viscoelastic carbon dioxide foam flooding system was studied in this paper. Based on the self-assembly characteristics of surfactants，the main foaming agent，auxiliary foaming agent，stabilizer and complexing agents were screened out in formation water without alkali and polymer. The formulation of 0.20% UCAB + 0.20% SDS + 0.09% triethanolamine + 0.08% EDTA was determined by single factor method. Under the optimized condition，the viscosity of the foam flooding system was 15.2 mPa?s，the foaming volume was 560 mL，the half-life was 58 min， and the improved foam composite index（IFCV）was 370272 mL?min?mPa? s. Metallographic microscope showed that the bubbles were irregular polygons such as pentagonal or hexagonal，indicating that the viscoelastic carbon dioxide foam had the general bubble characteristics. The viscoelasticity test showed that the small molecules formed wormlike micelles by self-assembling，and wormlike micelles entangled and associated with each other，macroscopically reflecting the viscoelasticity of polymer. Therefore， the foam flooding system not only met the injectivity，but also displayed the viscous and elastic oil displacement characteristics of polymer，which could enhance oil recovery effectively.