Abstract: In this paper, the decay and ingrowth code（DAIC）, which was verified by the burnup calculation code, has been developed by FORTRAN language; MELCOR was utilized to module the severe accident processes initialed by the large break loss of coolant accident（LBLOCA） and calculate the in-containment source term combing the DAIC code for AP1000NPP. Then, the influence of decay on the fission products release has been studied. The results showed that the decay has little influence on the release fraction of long half-time nuclides,such as ¹³¹I, ¹³¹mXe, ¹³³Xe, ⁸⁵Kr, ¹³⁴Cs and so on, whose release fractions are close to those discounting decay. The release fraction is on the rise as a whole along time; the decay has obvious influence on the release fraction of the short half-time nuclides, such as ¹³⁴I, ¹³⁵I, ¹³⁸Xe, ¹³⁸Cs, ⁸⁷Kr and so on, whose release fraction curves rise first and then decrease.