We use admittance spectroscopy to characterize the energy distribution of defects in CIGSe solar cells before and after annealing to investigate the mechanism of the annealing process improving the performances of solar cells. In this work, we anneal the prepared CIGSe solar cells in compressed air at 150 ℃ for 10 min. We measure dark I-V, C-V, admittance spectra, and illumination I-V tests on CIGSe solar cells before and after annealing to characterize the changes in the performances of solar cells before and after annealing, respectively. The test results of dark I-V characteristics show that the reverse dark current of CIGSe solar cell decreases by about an order of magnitude after annealing, and the ideal factor of the cell also decreases from 2.16 (before annealing) to 1.85 (after annealing). This means that the annealing process reduces the recombination of carriers in CIGSe solar cell. Under reverse bias, the capacitance of CIGSe solar cell is higher than that after annealing, and its C-V characteristics linearly fitted with 1/C ² vs. V. The fitting results show that the slope of the curve increases after annealing, which means that the annealing process results in a decrease in the free carrier concentration in the absorption layer of CIGSe solar cell, specifically, the carrier concentration contributed by defects after annealing decreases. In addition, the built-in potential before and after annealing of CIGSe solar cell are also obtained through fitting, which are 0.52 V and 0.64 V in value, respectively. The admittance spectrum test results of CIGSe solar cell before and after annealing show that the defect activation energy in the absorption layer significantly decreases after annealing, but the defect concentration remains almost unchanged. The decrease in defect activation energy means that the Shockley Read Hall (SRH) recombination probability of defects in copper indium gallium selenium solar cell decreases. In addition, the test results of the optical I-V characteristics of the battery indicate that the open circuit voltage and parallel resistance of the solar cell significantly increase after annealing, which is consistent with the test results of the dark I-V characteristics, C-V characteristics, and admittance spectroscopy of the solar cell. Therefore, the annealing process of CIGSe solar cells leads to theweakening of the SRH recombination of carriers in the absorption layer of the cell, thereby improving the performance of the solar cell.