A organic-inorganic composite membrane sealing agent was synthesized using modified nano SiO2 as rigid particles, acrylamide, 2-acrylamido-2-methylpropane sulfonic acid, styrene, and vinyl acetate as polymerization monomers, OP-10 and sodium dodecyl sulfonate as emulsifying agents, and ammonium persulfate and sodium bisulfite as initiators. The morphology and structure of the product were characterized using FTIR, TGA, DLS, and SEM. The effects of the sealing agent on the rheological properties, filtration properties, and sealing performance of water-based drilling fluids were studied. The results indicate that the composite film-forming sealing agent is spherical and maintains a nanoscale distribution. At a temperature of 340 ℃, the mass loss is approximately 14% of the total mass. After aging at 120 ℃ for 16 hours, the viscosity performance increases compared to before aging. When the amount of sealing agent added is 3% (based on the total mass of the system, the same below), the API filtration loss is 10mL. When the dosage of sealing agent is greater than 2%, the filtration loss of the sand bed is 0mL. When the dosage is 4%, the high-temperature and high-pressure filtration loss is reduced from 104 mL to 29mL, and the sealing rate is 90.7%.