In this work, the changes of irradiation defect in high dose (1.17×1019ncm-3) neutron-irradiated Czochralski silicon (CZ-Si) at different annealing temperatures was investigated using the Positron annihilation spectroscopy (PAS) and Fourier transform infrared spectrometer (FTIR) technique. It is found that divacancy (V2) and V4 abound in irradiated CZ-Si. With the increase of the annealing temperature the monovacancy type defect (such as VO) is annihilated. The results of PAS and FTIR show that the formation of V4 is enhanced when the annealing temperature ran up to 400-600℃ and with the FTIR absorption peak at the wave number of 829 cm-1 (VO) disappearing, five absorption peaks appear at the wave number of 825cm-1 (V2O2), 834 cm-1 V3O2; 840cm-1 V2O; 720cm-1 and 919cm-1: It can be concluded that these defect-impurity complexes prolong the lifetime of positrons.