The medium wave HgCdTe photovoltaic detector (band gap ～0.33 eV) is irradiated by a CW band-in laser. The experimental results are shown that the detector enter into nonlinear response state as the incidence laser power increase. When the detector entered into nonlinear response state, the open circle voltage (Voc) signal decreased with the laser power. The Voc signal rapidly decreased when the laser turned on and the Voc signal rapidly increased when the laser turned off. The effect of laser induced temperature field and temperature dependence built-in field of pn junction were considered. The laser power variation of laser turn on and turn off were aslo considered. The analytical model of photovoltaic detector irradiated by above-band gap CW laser was built up. The calculation results agree well with the experiment results. It is shown that the temperature dependence built-in field of pn junction is the main mechanism induced the nonlinear response. The magnitude of Voc signals with laser turn on and turn off is decided by light intensity and temperature.