Abstract: A simulation program was designed for finishing real-time simulation of nuclear-heat-release in out of pile experiment. Nuclear reactivity simulation program, a subprogram of simulation program, utilized three main models to simulate Doppler effect and moderator temperature effect, that was point-reactor model for neutron dynamic, one-(dimensional) homogenous flow model for hydrodynamic and one-dimensional transient heat conduction model for fuel dynamic. Two experiments, raising power and lowering flow, were performed used that program. Experimental results show that the program can meet the requirement of real-time simulation of nuclear-heat-release.