In the prospecting and drilling processes of gas and oil, if added into the drilling fluids, the latex nanoparticles can improve the performance of the drilling fluids. In this paper, methyl methacrylate (MMA) acts as a monomer and the combination emulsifier is a certain proportion of OP-10, RH and T-80. PMMA latex nanoparticles have been polymerized through the method called microemulsion polymerization. The conditions of methyl methacrylate microemulsion polymerization have been discussed and optimized. The dosage of emulsifier is 5 wt.%, polymerization temperature is 75 °C, and the stirring rate is 100 rpm. The particle shape, granularity distribution and molecular weight have been characterized by transmission electron microscope, laser particle size analysis device and gel permeation chromatography, respectively. The results indicate that the PMMA particles are ball like, showing average particle size of 65 nm, and that the MW is about 437,140. Results show that PMMA latex has strong inhibitive function on the shale swelling. When the latex is added into the drilling fluids, the filter loss decreases dramatically; and the PMMA latex drilling fluids system can be used when the temperature is below 180 °C.