Once fuel failures occur in a reactor core, the fission gas will release from the UO2 pellet to the fuel-to-sheath gap and eventually enter the primary coolant through the cladding wall breach. The crew of nuclear power plant regularly sample the primary coolant and characterize the fuel integrity based on the fission gases release model. If the activity of the primary coolant is too high and exceeds a certain limit, the degassing operation will be performed by the crew, causing a significant variation of primary coolant activity. As a result, the influence of degassing on the fission gases release should be taken into consideration in such situation. In this work, a modified model of fission gases release considering the degassing effect was established based on the fission product release mechanism and the authentic degassing processes of a nuclear power plant. The modified fission gases release model with degassing consideration was then validated by the measured primary coolant activity in a nuclear power plant. Finally, the model was applied on characterizing the fuel integrity of a fuel cycle with defective rods and frequent strong degassing operation. The dependence of release-to-birth ratio on decay constant of fission gases was determined using the modified model and the defect size was predicted. The predicted defect size was in good agreement with the sipping test result.