As a Generation-III PWR nuclear power plant (NPP) developed by China National Nuclear Corporation (CNNC), HPR1000 fulfils the international utility requirements for advanced LWRs. AHPR1000 is developed as an upgraded and innovative NPP based on HPR1000, which has a significant characteristic of combined active and passive safety design. An integrated test facility (ITF) with a 1:3 height ratio, 1:90 volume ratio and 1:52 power ratio by H2TS scaling analysis method, compared with AHPR1000, was designed and built to identify important thermo-hydraulic phenomena. The study of the natural circulation characteristic of the main loop reveals the system's residual heat-carrying capacity when the main pumps stop under loss of external power supply conditions. Aim to explore the limit range of feasibility parameter matrix under stepwise core power. Testing the limitations under natural circulation for equipment such as core electric heating rods. The coupling characteristics of the primary and secondary circuits are obtained. The pre-analysis before the test is vital. In this paper, a numerical model based on RELAP5 of the ITF was analyzed. The multistep core electric rod thermal power is considered to avoid flow instability and pressure fluctuation caused by flow pattern transition. Similar to an impassive NPP, residual heat is transferred by steam generators (SG) under shutdown conditions, which means SG simulate the passive residual heat removal system (PRS) functional, adopted in the simulation. An appropriate secondary side-referenced pressure setpoint is obtained, meanwhile, the feedwater mass flow rate value can be found by numerical simulation and heat transfer equilibrium theoretical calculation.