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.