The γ irradiation effects of long-wavelength HgCdTe photovoltaic detectors with different passivate structures(one structure was a single layer of ZnS and the other was double layers of CdTe/ZnS) were studied. A real-time measurement of current-voltage (I-V) characteristic was performed during the irradiation process. It is found that the detectors exhibit different irradiation effects as the irradiation dosages increase. Numerical fitting of the resistance-voltage (R-V) curves of detectors was made according to the current mechanism of photovoltaic detectors. It is found that the current is mainly trap-assisted-tunneling current(TAT) at larger reverse bias voltages. And it is generation-recombination current (G-R) at smaller and zero reverse bias voltages. After the analysis of R-V curves of detectors before and after irradiation, it is discovered that the double layers of CdTe/ZnS help to reduce the irradiation displacement effect and bring no obvious increase of indirect trap-assisted-tunneling current as the increase of irradiation dosage. It is also found that irradiation ionization effects are closely related to the initial material parameters of the detectors. Besides, it is also found that the detector with a single layer of ZnS has a higher generation-recombination lifetime of minority carriers by fitting method, and its generation-recombination current increases persistently as the increase of irradiation dosage.