Abstract: Theoretical research on soft-damage in photovoltaic semiconductor detectors under medium power density laser irradiation (between the saturation threshold and the damage threshold of detector) was carried out and a new mechanism was proposed. When laser irradiation density exceeds the saturation threshold of a detector, the transitions of the carriers between bands saturated, hot carriers are produced in the semiconductor and the hot carrier temperature is higher than the crystal lattice temperature. Therefore the output of the photovoltaic detector decreases with the increase of laser power density till zero-voltage output appears. The output of a photovoltaic HgCdTe detector under laser irradiation is calculated. The calculating results are identical with experimental results.