Since the Three Mile Island accident happened in the United States in 1979, the nuclear industry has accelerated the development of calculation codes for simulating severe accident. Especially after the Fukushima nuclear accident, the nuclear industry and nuclear safety regulatory authorities have paid more attention to the study of severe accidents in nuclear power plants. Based on advanced pressurized water reactors in Generation II, typical severe accident scenario, like small break loss of coolant accident (SBLOCA) is selected, and verification calculations on the Program Integrated for Severe Accident Analysis (PISAA), the severe accident integrated code developed by China with MELCOR, is conducted in this paper. The results of flow rates in break, pressure, water level and hydrogen production are obtained, showing that PISAA has the same trend compared with MELCOR reasonably, and PISAA has a completed calculation function which can simulate severe accident phenomena. Uncertainty analysis is also performed, 7 representative parameters related to hydrogen production are selected, and Latin hypercube calculation is performed. Hydrogen production distribution is around 300kg∼520kg, indicating that there is no cliff effect in PISAA. In the future, the researchers plan to do some works to improve the accuracy and functions of PISAA.