In the event of a postulated severe accident in the spent fuel pool (SFP), the cooling function of the SFP is weakened or lost, which leads to the inability to properly dissipate the decay heat from the spent fuel, causing the water level to drop until the fuel assemblies become exposed. Consequently, the cladding temperature of the fuel assembly gradually increases until it ruptures, releasing radioactive materials and potentially leading to external leakage. In engineering practice, the lumped-parameter accident analysis program is generally used to simulate the severe accident process in spent fuel pool. However, the phenomena of severe accidents and the processes such as the release, transport, and deposition of radioactive materials are extremely complex. The models of existing representative severe accident analysis softwares have large uncertainties. With the rapid development of computer technology and mathematical tools, the best estimate plus uncertainty analysis method is now more and more popular to be used in the field of severe accident analysis to reduce the uncertainty of the analysis results. Taking a certain spent fuel pool as an example, the best estimate plus uncertainty analysis method is introduced and adjusted to conduct uncertainty analysis of source term during severe accidents. From the perspective aerosol behaviors, the uncertain parameters are screened, the range and distribution of the uncertain parameters are determined by consulting relevant experimental databases, literatures, and program user manuals. An appropriate sampling method is chosen to sample the uncertain parameters to generate an uncertain parameter input matrix. The Wilks formula is applied to determine the number of sampling times that can not only meet the requirements of uncertainty analysis but also ensure calculation efficiency. Then batch calculations with different uncertain parameter values are performed to analyze the impact of parameters on source term, the uncertainty analysis results show that the selection of uncertain parameters has great influence on source term. Finally, an appropriate sensitivity analysis method is used to conduct sensitivity analysis on the uncertain parameters, and the ranking of the importance of the uncertain parameters to the source term is obtained, and the parameters that have a greater impact on the source term of severe accidents are determined.