In order to improve the problems of high temperature fracture，poor salt resistance and long-term stability of polymer flooding system in high temperature and high salt reservoirs，a new nano-dispersion system composed of oleic acid modified nano-SiO₂（O-SiO₂）and hydrophobically associating polymer KAPAM was developed. The viscosity，temperature resistance，salt tolerance，shear resistance，long-term stability and oil displacement effect of the nano-dispersion system were evaluated. The results showed that compared with single KAPAM solution，the three-dimensional grid structure of nano-dispersion system increased structural stability and structural resilience of the system，which improved the properties of the system. O-SiO₂ could increase the viscosity of KAPAM solution. The viscosity of nano-dispersion system composed of 0.15% KAPAM and 0.3% O-SiO₂ was the highest，and the viscosity increasing rate was 25.73%. The viscosity retention of nano-dispersion system at 90 ℃ was 89.41%. When the mass concentration of NaCl and CaCl₂ in the system was 60 g/L and 7 g/L，the viscosity retention was 72.13% and 65.74%， respectively. The nano-dispersion system could be stably dispersed for about 20 days at 90 ℃. Under the condition of 90 ℃ and salinity of 3.3×10⁴ mg/L，injecting 0.5 PV nano-dispersion system into sand filling pipe could improve oil recovery by 23.5% on the basis of water flooding，which showed good temperature resistance and salt tolerance，long-term stability and oil displacement ability.