The　water-soluble　copolymer　of　maleic　anhydride　(MA),　acrylic　acid　(AA)　and　sodium　methyl　allylsulfonate　(MAS)　is　prepared　by　free-radical　polymerization.　In　the　preparation　process,　water　is　used　as　solvent　and　sodium　persulfate　is　used　as　initiator.　The　effects　of　the　ratio　of　three　monomers,　the　dosage　of　initiator,　reaction　temperature　and　reaction　time　on　the　conversion　rate,　mean　molecular　mass　and　scale　inhibiting　result　of　the　copolymer　are　studied.　It　is　shown　that,　the　copolymer　has　excellent　performance　when　the　molar　ratio　of　MA　to　AA　to　MAS　is　3.0　:　1.0　:　0.6,　the　dosage　of　the　initiator　is　14%　of　the　mass　of　the　monomers,　the　reaction　temperature　is　90°C　and　the　reaction　time　is　4.0　h.　Under　the　optimum　conditions,　the　conversion　rate　of　the　monomers　could　reach　to　90.1%,　and　the　inhibition　rates　of　the　copolymer　to　CaCO3　and　Ca3(PO4)O2　could　reach　to　98.2%　and　92.1%　respectively.