Cloud points of the compressed CO2 + tetrahydrofuran (THF) + polystyrene (PS) system was studied at temperature range of 298.15-328.15 K and at different pressures. The expansion of the solution caused by dissolved CO2 was also studied. Synchrotron radiation small-angle X-ray scattering (SAXS) was used to investigate the dissolved CO2 on the conformation of PS. The second virial coefficient A(2) and the apparent mean-square radius of gyration [R-g(2)](1/2) at different pressures were obtained. It was found that the PS chain shrinks with increasing the concentration of CO2.