In the WIMSD-IAEA multigroup nuclear data library, the isotopes and weights adopted for WLUP libraries to calculate the average fission spectra for 235U, 238U, and 239Pu are in the ratio of 54%, 8%, and 38%, respectively. The average fission neutron spectrum in the existing multigroup WIMSD-IAEA library applicable for the U-Pu cycle is not rigorously applicable for systems that are based on the thorium fuel cycle because of two aspects. First, the weightage of the fission neutron spectrum of 232Th and 233U nuclides, which are important isotopes in the thorium fuel cycle, are not considered in obtaining the average multigroup fission spectrum in the conventional WIMSD-IAEA library. Second, the 232Th/233U system spectrum is required for condensation of the fission spectrum as done in generating other multigroup cross sections and parameters for the thorium fuel cycle. In this work, we have processed the fission neutron spectrum data from the basic evaluated nuclear data file (ENDF/B-VI.8) for each important isotope in the thorium fuel cycle using the Th/233U spectrum and using a FORTRAN program developed and validated by us for this purpose. The final average fission spectrum to be fed into the WIMSD-IAEA library is prepared by mixing the isotopic multigroup fission spectrum of individual isotopes 233U, 239Pu, and 241Pu with appropriate weights corresponding to their respective power fractions in the advanced heavy water reactor (AHWR) lattice. Using the WIMSD library with modified effective fission spectra, the lattice k-infinity calculations of AHWR are performed as a function of burnup. The difference in the infinite multiplication factor, which is expressed in terms of reactivity in mk, ranges from 0.48 to 0.94 mk as burnup in the AHWR proceeds from 0 to 55 GWd/tonne.