The invention provides a specific-antigen preparing method used for detecting schistosoma japonicum antibodies by an indirect hemagglutination (IHA) method. The method includes the following steps: step one, preparing schistosoma-specific antigens, wherein in a step 1, soluble adult antigens of schistosoma japonicum are prepared, in a step 2, soluble cercaria antigens of the schistosoma japonicumare prepared, in a step 3, soluble egg antigens of the schistosoma japonicum are prepared, in a step 4, the antigens are treated by sodium periodate, and in a step 5, the soluble adult antigens of theschistosoma japonicum, the soluble cercariae antigens of the schistosoma japonicum and the soluble egg antigens of the schistosoma japonicum after treating of the sodium periodate are proportionallymixed according to a volume ratio of 1:0.5:2, that is, specific antigens are obtained; and step two, preparing an indirect hemagglutination reagent by the specific antigens. According to the method, the crudely prepared soluble antigens of schistosoma japonicum adults, cercariae and eggs are proportioned and mixed after treating of the sodium periodate, the diagnostic reagent is prepared, and thereagent reduces cross-reaction with other diseases, and maintains original high sensitivity and specificity at the same time.