Since 2012, China’s provinces have gradually introduced a serious illness insurance（SII）in the new rural cooperative medical system（NCMS）. Based on the micro-household survey data and the multiple difference method, this paper empirically studies the impact of major illness insurance on the health and medical expenditure of rural residents on the basis of correcting the heterogeneity of self-assessed health reports. The results confirm that more generous medical insurance has increased the improvement of rural residents’ health by 10%-20%, and this effect is still significant in the third year after the implementation of the policy. SII has also stimulated a substantial increase in the medical expenditure of rural residents, resulting in a 5% increase in per capita household medical expenditure and 29% increase in individual hospitalization expenditure, respectively. But these effects differ among different populations. From the perspective of the health improvement effect, rural middle and upper income families are the biggest beneficiaries. From the perspective of the incentive effect of medical expenditure, the incentive effect of SII increases with the increase of family incomes. The results of the conditional quantile DID model show that SII has significantly stimulated the expense of the group with higher medical expenses, and the group with less medical expenditure has not been influenced by SII. A series of robustness tests prove the robustness of our results. The conclusions of this study provide an empirical basis for further improving the basic medical insurance system in China.