Abstract: Objective Toanalyze the inter vention effect of semiconductor laser on endodontic interappointment emer gencies(EIA E).Methods 304 teeth which required root canal therapy for apical periodontitis(acuteor chronic)or pulp necrosis were randomly divided into 2 groups.In the first group(n=155),the root canals were filled with CP after root canal preparation.A nd in the other goup(n=149),the apical points outside the teeth were irr adiated with semiconductor laser after the root canals were filled with CP.In addition,175 teeth suffering from EIA E after obturation were investigated and divided into 2 groups.One group(n=96)was irradiated by semiconductor laser on the apical points outside the teeth.T he pat ients whose teeth in the other group(n=79)were informed totake medicine(Ar ilin and Cephalosporin).A nd 4 hours later,the sy mptom and clinical examination of all the patients wer e recorded.Results The rate of 304 EIA E after root canal pr eparatione was 4.52% in the group irradiated with semiconductor laser and 101 74% in the other group(P<0.05).The therapy effcct of semiconductor laser was better than that of taking medicine on EIAE(P<0.05).Conclusion Semiconductor laser can decrease EIA E,and it could be used for the treatment of EIAE.