The photovoltaic HgCdTe detectors with band gaps 0.91 and 0.33 eV are irradiated by 10.6 m laser (0.12 eV photon energy), separately. It is found that output voltage of detector (0.91 eV band gap) is positive, while the response voltage of detector (0.33 eV band gap) is opposite to it. To investigate this phenomenon, the detector with a band gap of 0.91 eV is irradiated by a given power 10.6 m laser under different initial open-circuit voltags. It is experimentally demonstrated that the phenomenon is caused by the initial open-circuit voltage. With further investigation, the open-circuit voltage of the photovoltaic detector is determined by both the thermovoltage caused by thermoexcited carrier and the crystal thermal effect produced by free carrier absorption under sub-bandgap laser irradiation.