Abstract: The thermionic reactor power system TOPAZ-Ⅱ of former Soviet Union was simplified. The core thermal-hydraulic model, neutron physics model and heat rejection system model were established, respectively. One-dimensional thermal-hydraulic model was used to model the coolant circuit. Two-dimensional analysis models for the core heat transfer and the radiator fin heat conduction with radiation were established. The point reactor kinetics equation with six-group delayed neutrons, taking the temperature feedbacks of UO₂ fuel, thermionic electrode, moderator and reflector and the control drum reactivity control into account, was applied. The changes of temperature, power and reactivity feedback of TOPAZ-Ⅱ under normal start-up procedure were obtained. The results are in good agreement with the corresponding results of TITAM code. Meanwhile, the developed code can be more precise in analyzing the start-up of TOPAZ-Ⅱ, and provide reference for the design of the control system.