In view of water sensitive heavy oil reservoirs，inhibiting the expansion of clay is the premise for effective thermal recovery. However，the mechanism of anti-clay swelling is remaining unclear after steam injection. Therefore，a scheme of inhibiting clay swelling by using Pickering microemulsion was proposed. Through the combination of Fick’s law and Langmuir adsorption theory，the mechanism of anti-clay swelling by Pickering microemulsion was quantitatively analyzed. The Pickering microemulsion was prepared by mixing hydrophobic nano SiO₂ with lauryl three methyl ammonium bromide（DTAB）. The surface tension of the microemulsion and the anti-bentonite swelling rate were determined. Then the mass ratio of SiO₂ and DTAB，and the amount of microemulsion were optimized. Additionally，the microemulsion flooding was carried out according to the best formula. The quantitative analysis of mechanism showed that it was feasible to use surface adsorption to prevent clay swelling. The best anti-swelling effect was obtained when the mass ratio of nano SiO₂ and DTAB was 1∶2 and the dosage of microemulsion was 0.6%. The best injection volume of microemulsion was 0.2 PV in sand pack flooding experiment. With increasing injection volume of Pickering microemulsion，the anti-swelling effect was enhanced，but the rate increment of water cut in outlet during sequent water flooding was also accelerated. Therefore，clay expansion should be inhibited properly in heavy oil reservoir with high permeability.