With the development of infrared detector technology, extending the operating wavelength of the detector has become one of the important development directions. Because the operating wavelength of the detector varies with the x value, mercury cadmium telluride (HgCdTe) materials have been widely used. As the operating wavelength increases, dark current becomes a major factor limiting the development of infrared detectors. Taking a 12.5 μm HgCdTe long-wave infrared detector as an example, the effects of operating temperature and fixed charge on its dark current are studied by simulation, and different types of dark currents at 77 K and 60 K are analyzed. This research gives us a deeper understanding of the dark current of 12.5 μm detectors, and provides a direction for improving the preparation level of 12.5 μm detectors.