In view of the current domestic problems of fewer types of polymer fluid loss additives for oil-based drilling fluids. Choosing Acrylamide（AM）、Stearyl methylacrylate（SMA）as reaction monomers, 2-methylpropionitrile（AIBN）as initiator, Divinylbenzene（DVB）as crosslinker. The oil based fluid loss additive was synthesized by dispersion copolymerization. Single factor method is used to determine the best synthetic condition of fluid loss additive. Through measure the APT filtrate losses determine the monomer ratio of AM:SMA=1:3, the monomer concentration is 5%, the crosslinker concentration is 2%, the initiator concentration is 0.7%, V（methanol）:V（H2O）=7:3, the stabilizer concentration is 4%, polymerization temperature is 70℃, polymerization time is 6h. Fourier transform infrared spectroscopy shows that the structure of the product is consistent with the design, the glass transition temperature is 110℃ through DSC measure. When the dosage of fluid loss additive is 3%, the API fluid loss is 1.6mL, HTHP fluid loss is 7.2mL,The high-temperature aging test show that the fluid loss additive has the temperature resistance of 160℃. the result show that the synthesized product has better performance as oil based fluid loss additive than asphalt and have no effect on drilling fluids’ rheology, and the quality of mud cake is better than the asphalt under the same dosage.