Nonionic nano-micro latex was prepared via emulsion polymerization with styrene (St), butyl acrylate (BA) and polyethylene methyl ether methacrylate (MAPEME). The structure of latex was characterized by Fourier transform infrared spectroscopy (FTIR). Particle size tester was employed to detect the size distribution and 20% volume of latex was attributed to nano-scale. Gel permeation chromatography (GPC) testing result showed that the average molecular weight of latex was 403,493 with polymerization dispersion degree (Mw/Mn) was 2.30. The influence of latex on filtration performance in water-based drilling fluids was estimated, and swelling tests were carried on both under room temperature and normal pressure (RTNP) and high temperature and high pressure (HTHP) to evaluate the interaction between the bentonite clay and latex. Experimental data showed that the latex controlled the HTHP filtration of polysulfide drilling fluid and KCl/polymer drilling fluid to 10-15 mL at 120 degrees C by improving the quality of filter cake. The latex could relieve the hydration expansion of the bentonite clay balls and keep ball integrity for 7 days under RTNP or 16 h at 90 degrees C under HTHP. In addition, the presence of NaCl was helpful to maintain the stability of the bentonite clay balls till 120 degrees C.