The invention discloses granule-bound starch synthase. The synthase mutates in one or more amino acid sites corresponding to an amino acid sequence shown in SEQ ID NO:1, wherein the sites are selected from a lower group and are H236, N265, Y268, N353, R408, E410, K413, C487, A114, EA314-315 and T543. The enzyme specific activity of the synthase is reduced. Thus, the relation between specific amino acid residues and the amylose content is disclosed, and therefore the content of amylose in rice can be adjusted, and then the granule-bound starch synthase can be used for improving the edible quality of rice.