Abstract: Reflood phenomenon is the third phase of large break loss of coolant accident（LBLOCA）, and it is also the most important stage to prevent it from developing towards severe accident. In order to improve the calculation accuracy of reflood with RELAP5 code, this paper performs sensitivity analysis against constitutive models of various flow regimes involved in the phenomenon. Then the model parameters which have a great influence on the results of interests including peak cladding temperature（PCT） and quench time are selected, and the improvement of these sensitive models are carried out with the help of recent advanced researches. FEBA（Flooding Experiment with Blocked Arrays） facility is used as the assessment experiment of this study. Through the contrast with six typical tests of FEBA experiment, an effective improvement of prediction accuracy is proved for the modified models.