Styrene-isoprene-styrene block copolymer (SIS) can improve its physical and chemical properties and mechanical properties through catalytic hydrogenation, making it more widely used in industry. Here a simple chemical reduction method was employed for preparing Pt/g-C3N4 catalyst using chloroplatinic acid and urea as raw materials, and the influence of catalyst composition ,the amount of Pt and reaction conditions on the hydrogenation performance of SIS was investigated. The structure of catalysts and products was characterized by XRD, TEM, 1HNMR, DSC, etc. The results shows that Pt/g-C3N4 exhibits a superior activity with 85% hydrogenation degree of SIS and 95% selectivity to C==C bonds of isoprene units under the conditions of 140 ℃, 3 MPa, 2 h, with cyclohexane as solvent.