Abstract: A versatile reactor system analysis code RESYS for advanced nuclear reactor was developed by using object-oriented language C++. Based on RESYS code, the model of thermionic nuclear reactor TOPAZ-Ⅱ developed in the former Soviet Union was established, and its start-up process was simulated. The established TOPAZ-Ⅱ reactor system model included reactor core thermal model, thermionic conversion system model and the radiator model. The current density model of cesium thermionic converter used model by Rasor. And the 6 groups of delayed neutron point reactor kinetic model was used to consider the influence of various structural components on reactivity. The point reactor equation with strong rigidity was solved using the Gear algorithm in the RESYS code. The reactivity feedback model of TOPAZ-Ⅱ reactor took into account the Doppler reactivity feedback of the fuel, the reactivity feedback of the electrodes, the reactivity feedback of the moderator and reflector, and the introduction of reactivity control drums. The steady state and start-up transient were simulated based on established thermal-hydraulic model and RESYS code. At steady state, the maximum temperature on the inner surface of the central thermionic fuel element is 2 291 K, the maximum temperature on the outer surface is 2 066.55 K, the maximum temperature on the emitter is 1 961 K, the maximum temperature difference between the electrodes is close to 1 200 K, the temperature at the coolant inlet is 743 K, and the temperature at the core outlet is 837 K. The comparison between the steady-state operating conditions calculated by the RESYS code and the TOPAZ-Ⅱ design values was carried out. The calculated steady-state electric power output is consistent with the calculation results of TITAM code. The calculation results verify the correctness of the developed RESYS code and the established TOPAZ-Ⅱ system model. During the startup process, the reactivity feedback of the moderator and reflection layer is positive feedback, which dominates all feedback effects. The overall reactivity feedback of the TOPAZ-Ⅱ reactor core is positive and does not have a self-stable negative feedback effect, which increases the difficulty of reactor control. During the start-up process of TFE, due to changes in the gas thermal conductivity of the electrode gap, the temperature of the fuel pellets and emitter rapidly increases. At this time, some of the thermal energy generated by fission is stored in the fuel pellets, causing a decrease in coolant temperature and the power of the radiation radiator.