Abstract: Decay heat is extremely important in core design and safety analysis. At present decay heat calculations are usually based on industrial standards or professional computer codes. Using the two methods to calculate the PWR assembly irradiation respectively, the validation was done by comparing the results with the two methods and the results were reasonably close. The analytical work was carried out on specific contributions made by important isotopes to decay heat at different shutdown time and the main contributors were screened out as a reference for the core source term inventory selection. The result shows that about 50 isotopes can bound 95% decay heat during 100 h50 a after shutdown. Discrepancies between the standard and computer code were also investigated. Some instructive advices on suitable region were given for applying the standard.