In order to study the primary influencing factors of jet breakup length during the core melt jet breakup process and their ranking,25 sets of experiments with 3 factors and 5 levels were designed based on the orthogonal experimental method,and the jet breakup lengths were obtained for each condition by using the Moving Particle Semi-implicit method(MPS).The simulation results were analyzed by polarity and variance analysis,and it was concluded that the primary influencing factors of jet breakup length were jet to coolant density ratio,jet velocity and jet diameter,and all of them have a second level of significance(**).Their ranking in the same level was in the following order:jet velocity>jet to coolant density ratio>jet diameter.Moreover,a new empirical relationship for predicting jet breakup length was fitted using simulated data,and the error margin of the model was kept within±30%when the ratio of jet to coolant density was 1.1~13.6.