In agricultural production, temperature and moisture are important factors affecting grain yield and quality.  Although moderate drought at the grain-filling stage can effectively alleviate the damage caused by high temperature, the specific regulatory mechanism driving the effect of moderate drought at the high temperature on starch synthesis is still unclear.  To explore the effects and mechanisms of high temperature and moderate drought on rice starch synthesis at the grain-filling stage, the activities of enzymes and expression levels of the genes involved in starch synthesis under four different treatments involving high temperature and/or water stress (CK, HT, WS, and HT+WS) were investigated in this study.  The starch synthesis of a japonica inbred rice was measured under the four treatments during the grain filling.  The results show that the effects of high temperature and moderate drought on grain filling mainly occur in the inferior grains of rice.  Through the regulation of enzymes involved in starch synthesis and the expression levels of their main genes, the synthesis of rice starch can be affected.  Therefore, the high temperature and moderate drought were antagonistic, and moderate drought can alleviate the damage to grain quality at a high temperature by improving the starch synthesis of inferior grains in japonica rice.  This study provides a basis for stress-resistance cultivation and breeding strategies of rice with high temperature tolerance.