HgCdTe avalanche photo diode (APD) is one of the developing trends of third generation infrared FPA detectors. This article presents a new method to fabricate HgCdTe APD by ion beam etching (IBE), and discusses the relation of gain to cutoff wavelength and depletion region thickness. A gain of 1000 at a bias of 17 V and a cutoff wavelength of 4.8 μm was achieved. The excess noise factor, F, is calculated after a noise spectrum test.