Series styrene-alt-maleic anhydride (SMA) copolymer with different molecular weight were prepared by solution polymerization, the structure of which were characterized by FTIR, 1H-NMR, 13C-NMR. And its thermal property was analyzed by DSC. The effects of molecular weight of SMA to its retanning-filling properties in leather was studied. The results show that the molecular weight of copolymer is an important factor to affect the retanning-filling properties. With the molecular weight increasing, the tear strength and reduction rare of location difference of leather are increasing, while the tensile strength decreasing. When the Mn of SMA is 6620, the tensile strength, tear strength and reduction rare of location difference are 18.325 N/mm, 9.519 N/mm2 and 7.44%; When the Mn is 59058, the tensile strength, tear strength and reduction rare of location difference are 20.577N/mm, 5.539 N/mm2 and 44.98%.