Emulsification and viscosity reduction is one of the key techniques for enhancing the recovery efficiency of heavy oil. Due to the inability of conventional viscosity reducers to meet the high temperature and high salinity requirements during thermal recovery of extra-heavy oil，issues such as viscosity reducer deactivation，poor viscosity reduction performance，and low recovery rates often arise. Using nano-SiO₂ sol，amphiphilic dispersant，modifier silane coupling agent，and synergist amphiphilic surfactant as raw materials，a nano-SiO₂ viscosity-reducing oil displacement agent was prepared. The microstructure of the oil displacement agent was characterized. Its viscosity-reducing effect on extra-heavy oil，temperature and salt resistance，adsorption performance， and oil displacement efficiency were evaluated. The results showed that the formula of the oil displacement agent was obtained as follows：50% SiO₂，40% dispersant，4.2% surface modifier and 5.8% synergist. The particle size was about 50 nm with uniform distribution and good dispersibility. The viscosity reduction rate of 1% original oil displacement agent for extra-heavy oil reached 84.99%. After aging for 2 hours at 350 ℃ and a salinity of 15 g/L，the viscosity reduction rate was 83.62%，maintaining 98.39% of its performance. The oil displacement agent changed the sandstone surface from hydrophobic to hydrophilic. Under the condition of 50 ℃，a liquid-solid ratio of 10∶1，and an equilibrium adsorption time of 15 hours，the static adsorption capacity of the oil displacement agent on oil sand was 1.377 mg/g sand. The dynamic adsorption capacity of 1% agent in a packed sand tube was 0.745 mg/g sand，indicating low adsorption loss in the reservoir，which met the requirements for chemical flooding. In a one-dimensional displacement experiment，the oil displacement efficiency of the agent was 26.7 percentage points higher than that of conventional steam flooding. After aging at 350 ℃ for 2 hours，the oil displacement performance retention rate was 97.0% . The nano-SiO₂ viscosity-reducing oil displacement agent had excellent viscosity reduction， temperature and salt resistance， adsorption performance and oil displacement efficiency，which effectively lowered oil recovery costs and then enhanced thermal recovery of extra-heavy oil.