Hg1-xCdxTe (x=0.3) avalanche photodiodes (APDs) with a PIN structure was investigated theoretically. The energy dispersion factor and the threshold energy are acquired according to the parameters of material. The gain as well as the breakdown voltage of the device was obtained. The composition, thickness, doping level were optimized theoretically for the APD device. A high performance APD device with a gain of 335 at the bias voltage of-10V was fabricated, which consisted of a PIN structure mad of HgCdTe grown by MBE.