The I-V characteristics of mid-wavelength HgCdTe photodiodes were studied under the variable incident illumination.The result shows that the photocurrent of photodiode increases with the increase of incident illumination,and the differential resistance of photodiode decreases.When the incident illumination is constant,the differential resistance of photodiode descends with the raise of reverse-bias voltage.The R-V curve of photodiode was fit by "lucky electron" model.It verifies that due to plenty of photo-generated carrier occurred in the depletion region,photocurrent multiplication induced by electron impact ionization is the primary reason of decreasing reverse-bias differential resistance.