PMA-MMA emulsion with high mass ratio of methacrylic acid (MA) and methyl methacrylate (MMA) was prepared using MA and MMA as monomers and ammonium persulfate as initiator by soap-free emulsion polymerization. The effect of different reaction conditions on polymerization was studied. The optimum reaction condition of PMA-MMA emulsion was obtained as follows: 70℃ reaction temperature, 6:1 mass ratio of MA and MMA, 13% total monomer mass fraction, 2‰ mass ratio of initiator and total monomer, 25 min?reaction time of MA and initiator and then MMA adding, 5 h continuous reaction time. The viscosity of the obtained white emulsion was 280 mPa·s, the conversion rate was 99.1% and gel rate was 0.5%, without separation after storage for one month at normal temperature. PMA-MMA emulsion was used to treat the oilfield oily wastewater in the east of South China Sea, the results showed that the oil removal efficiency could reach 95% when emulsion dosage was 15 mg/L, which was superior to that of on-site foreign reagent (92%). The oil content of discharged production waste water decreased from15 mg/L to 8 mg/L when the emulsion dosage on site was 10 mg/L.