Abstract: When LOCA happened, judging accident type at the beginning was quite important for the operator to safely operate nuclear power plant, thus, a method to judge accident type based on monitoring parameters was proposed. In this method, according to the mapping between the change rates of monitoring parameters and the accident types in the first 13 s after accident, the accident symptom was distilled and a judging model was built, with which accident types were judged by using SVM classify method. Test results show that the method can judge the break size and the relevant position of a typical LOCA exactly and effectively in the beginning of the accident.