Abstract: MOSAP (modular severe accident analysis program) is an integrated severe accident analysis code for nuclear power plants independently developed by Xi’an Jiaotong University. In order to verify the rationality and accuracy of MOSAP, it was used to model and calculate the nuclear accident process of Three Mile Island Unit 2 (TMI-2). In the model established by the MOSAP, the two cold pipe sections and two main pumps in each loop of TMI-2 were simplified into one cold pipe section and one main pump. The important emergency core cooling system was simulated in the primary loop system, and the pressurizer and its spray and heater devices were also simulated. The core area was divided into 16 axial segments and 4 radial rings. The lower head was divided into 5 sections in the radial direction and 5 layers in the thickness direction. The steady-state calculation results of the MOSAP are in good agreement with the TMI-2 system parameters, indicating that the models established by MOSAP can simulate the operation of the power plant well. This model was used for accident transient calculations and the calculation results of key parameters in the accident process such as the pressure of the primary loop system, the water level of the pressurizer, the water level of the core, the fuel temperature, the amount of hydrogen produced, and the mass of molten material in the lower plenum were obtained and compared with the measured accident values and the calculated values of the internationally recognized severe accident analysis program ASTEC (V2.2). The results show that the calculated values of the pressure of the primary loop system, the water level of the pressurizer, the water level of the core, and fuel temperature in the MOSAP are in good agreement with the measured accident values and the calculated values of ASTEC. The hydrogen production value calculated by the MOSAP is about 541 kg and the total mass of debris in the lower plenum is about 18 t, both of which are closer to the measured values of TMI-2 than those calculated by the ASTEC. ASTEC can’t be used to calculate the significant migration of molten material towards the lower plenum, which is due to the solidification of the molten material at the core lower plate caused by the certain water level held in the lower part of the core. Overall, the deviation between the calculated key parameters of MOSAP and the measured values of TMI-2 is within 20%. MOSAP can simulate the main processes and phenomena of severe accidents such as core overheating and cladding oxidation, core material melting, migration and relocation, core reflooding, and lower head response.