This paper models a device structure of n-on-p Hg1-xCdxTe infrared detector by means of Sentaurus TCAD. Currents in different electrode sizes are simulated. Through simulation it is found that reverse current of photoelectric diode gradually decreases with the decrease of the electrode size. This paper analyzes the phenomenon from two aspects,metal layer’s effects on the electric potential distribution and the number of photons(Q) absorbed. The metal layer has a modulation effect on electric potential distribution and Q. The two causeses result in that the current decreases with the decrease of electrode size.