Remission, just a temporaiy policy, had already existed in ancient times of China. But this institution became a regular policy in Song Dynasty, "every three years one remission on suburb sacrifices". As a result, it took on different characteristics than the former dynasties. The changes of the rulers' lawful ideas, the pressure from internal trouble and foreign invasion and the push fiiom scholar-bureaucract were the main reasons for Song Dynasty's great interest in remission. The remission institution mitigated the social contradictions of Song Dynasty and caused great harm at the same time.