Abstract: Thermal hydraulic calculation module RELAP5 is one part of digital reactor system(DRS).The modeling method by RELAP5 is described.There are mainly following 3 steps: The first is modification of RELAP5 source code;the second is constructing normal model;the third is establishing special model.Thus besides plant-running parameters can be dynamically extracted from RELAP5,it can be dynamically changed in RELAP5.This model is strong enough to meet the needs of nuclear power plant engineering simulator.