Abstract: In some severe accident scenarios of AP1000 nuclear power plant, a breach or bypass of the containment will lead to large release to environment, which causes severe radioactive pollution of environment. Three release categories (bypass, early containment failure and containment isolation failure) have the largest contribution to large release frequency. Three typical severe accidents (steam generator tube rupture, spuriously open of automatic depressurization system valves and reactor pressure vessel rupture) were selected as typical cases corresponding to the three release categories and the fissionproduct source terms released to environment were calculated using MAAP code. The analysis results are provided as essential input data for quantifying the release of AP1000 nuclear power plant and the offsite dose analysis.