A new phenomenon is observed when a photoconductive InSb detector with 0.228eV band gap is irradiated by 10.6 μm laser, whose photon energy is 0.12 eV. The detector is heated by this out-band laser, due to the absorption of laser energy. However, a transformation temperature T0 exists in this process. When the temperature of the detector, T, is lower than T0, the number of carriers remains constant but the conductivity changes because of a change in mobility. The mobility decreases with the increase of temperature and varies as T-2.35. At T>T0, the concentration of thermally-activated carrier increases with temperature, which is proportional to exp (-Eg/2k0T). As a result, the influence of carrier concentration becomes more and more important. As a result, the output of the detector decreases. In a word, the output voltage of photoconductive detector results from the temperature dependence of mobility and concentration of carriers. This work provides an experimental basis for improving the carrier transport model.