Background: Nonparametric statistical method is a kind of statistical inference method not depending on a certain distribution;it calculates the tolerance limits under certain probability level and confidence through sampling methods.The DNBR margin is one important parameter of NPP design

which presents the safety level of NPP.Purpose & Methods: This paper uses nonparametric statistical method basing on Wilks formula and VIPER-01 subchannel analysis code to calculate the DNBR design limits（DL） of 300MW NPP（Nuclear Power Plant） during the complete loss of flow accident

simultaneously compared with the DL of DNBR through means of ITDP to get certain DNBR margin.Results: The results indicate that this method can gain 2.96% DNBR margin more than that obtained by ITDP methodology.Conclusions: Because of the reduction of the conservation during analysis process

the nonparametric statistical method can provide greater DNBR margin and the increase of DNBR margin is benefited for the upgrading of core refuel scheme.