In order to develop the application field of polymers synthesized by dispersion polymerization，amphotericcopolymer which could be used as filtrate loss reducer was synthesized in ammonium sulfate solution by acrylamide（AM），acrylic acid（AA），2-acrylamido-2-methyl propane sulfonic acid（AMPS）and methacryloyloxyethyltrimethyl ammonium chloride（DMC）as monomers，polyvinylpyrrolidone（PVPK12）as dispersing agent，and 2，2'-azobis（2-methylpropionamide）dihydrochloride（V-50）as initiator. The effects of different concentration of ammonium sulfate，AMPS，monomers on the apparent viscosity and particle size of dispersions and intrinsic viscosity of polymers were discussed. The structure was characterized by FTIR and 1H-NMR，and the filtration reduction property of copolymer in bentonite slurry was evaluated. The results showed that the structure of polymers was consistent with the designed one synthesized on following conditions：15% monomers concentration，2∶5∶1∶2 monomer ratio of AM∶AA∶DMC∶AMPS，0.0426% V50 dosage（in terms of total monomers），1.4% PVPK12 dosage（in terms of total water），55℃ temperature，5 pH value. Polymers in dispersion were spherical and distributed uniformly which had a narrow diameter distribution. The dispersion could be stored beyond one year at room temperature. The ammonium sulfate and AMPS concentration had same effects on the apparent viscosity and particle size of dispersion. With increasing dosage of ammonium sulfate and AMPS，these two variables were decreased first and then increased. When the concentration of ammonium sulfate increased，the intrinsic viscosity number of polymer increased first and then decreased，while AMPS had the contrary effect on intrinsic viscosity number.With increasing concentration of monomers，the apparent viscosity and particle size of dispersion were both increased. The polymer had a good performance on filtrate loss reducing. With increasing dosage of polymers in bentonite slurry，the API and HTHP fluid loss decreased and became stable gradually. Amphoteric copolymer synthesized by dispersion polymerization in ammonium sulfate solution could be used as filtrate loss reducer in drilling operation.