Abstract: The Civil Code added a new cause of marriage cancellation, "concealing major diseases", to the revocable marriage system. The Civil Code did not clearly define the connotation of "major diseases", which has caused divergence in understanding between the academic and judicial circles. Through the analysis of the legislative evolution, it indicated that the provisions on diseases in marriage have gone through the process from public interest to personal interest. Major diseases included in the revocable marriage system reflects the importance of legislation on personal interests. Therefore, major diseases should be understood from the perspective of personal will. However, this does not mean that personal will is the only basis for identifying major diseases. Whether it refers to a major disease still needs to take into account the interests of the weak in the marriage and family relationship. In essence, this is a process of interest game, and the identification of major diseases needs to balance different interests. Thus, the definition of major diseases should be based on whether the damage to the interests of the concealed party caused by the disease is higher than that of the weak in the marriage and family relationship caused by the dissolution of the marriage. If it is higher, the court can regard the disease claimed by the concealed party as a major disease.