Abstract: The estimation of radioactive source term released to environment under severe accident for a nu-clear power plant using code package STCP consunes a large amount of computer time and expense. It isactually not practical to perform the source term code package calculations for all accident senarios ofinterest. For this reason a simplified calculation method is developed. Its essential idea is as follows: cal-culate sveral selected sequences using STCP, then treat and analyze the results according to the chacteris-tics of radioactive release and accident progress. Some special parameters can be deduced. The sourceterms for other accident sequences can be estimated by combination of there parameters.