The current-voltage (I-V) characteristics of HgCdTe photodiode under up to 33 dB of irradiance level have been investigated with a continuous-wave solid state laser. Short-circuit photocurrent and open-circuit photovoltage are observed to be saturated under high-level photon injection. It is found that the increasing of reverse bias can dramatically improve the response linearity of photodiode under high irradiance. The I-V characteristics and dynamic resistance versus bias relationship of photodiode under irradiance cannot be explained satisfactorily with present theory. Considering the effect of irradiance on photodiode series resistance and parallel resistance, an interpretation based abrupt junction model is presented to account for the effect of bias on I-V characteristics of HgCdTe photodiode.