Abstract: The code to simulate reactivity feedback during out-of-pile testing has been developed with C++ in the paper. Simulation of reactivity feedback, simulation of the system of power control and simulation of neutron kinetics are the main modules of the code. There are three main physical models, i.e., point-reactor model for neutron dynamic, one-dimensional homogenous flow model for hydrodynamic and onedimensional transient heat conduction model for fuel dynamic in the code. The scheme of simulation of power control system is established with a conservative average reactor temperature. The auxiliary calculation modules such as calculation of physical property parameter and calculation of geometric parameter are also taken into account. Finally, the data given by Retran-02 are selected to validate the present code and the results indicate that our code meets the requirement of real-time simulation of reactivity feedback.