Abstract: The abnormal responsive mechanism of photoconductive semiconductor detectors irradiated by off-band laser, hot electron photoconductivity, was proposed. The response of a photoconductive semiconductor detector under off-band laser irradiation is different from that under in-band laser irradiation. Heat effect in detectors was considered simultaneously under high power laser irradiation. The outputs of the detector were calculated accordingly. The calculated results show that the production of hot electrons causes the conductivity to decrease when a photoconductive HgCdTe detector is irradiated by off-band laser and that the resistance and the output of the detector are increased consequently.