Immersion gratings are typically used in the infrared spectral band in which optical materials with high refractive index. Some issues inevitably arise due to their special operating mode. And they are critical to the application of immersion gratings. This study investigated the relations between the lengths of the long- and short-wave of normal gratings. For the features of the relevance of the refractive index with wavelength, the short-wave infrared spectral band (1.5 similar to 2.5 mu m) was taken as an example to analyze the change characteristics of spectral line location and spectral resolution of immersion gratings. Results showed that when immersion gratings were adopted, significant differences were observed in the distribution of spectral lines as compared with normal gratings, and "trapezoidal" spectral lines tilted. For short-wave infrared spectral lines, the inclination of the "trapezoidal" spectral lines became more apparent due to larger changes in the refractive index of the medium. When Littrow conditions were registered with long-wave length (2. 5 mu m), the spectral line tilted toward the short-wave end. When registration was done with short-wave length (1. 5 mu m), the spectral line tilted toward the long-wave end, and the Littrow wavelength drifted away from, the central wavelength. The refractive index had a few changes at the thermal infrared spectral band. The tilt of "trapezoidal" spectral lines was smaller and more similar to that of normal gratings. The spectral resolution of immersion gratings varied due to changes in the refractive index. At the same order, the wavelength increased, and resolution increased; between various orders, the order decreased, and the resolution decreased. Meanwhile, considering that the high-order resolution was larger than the low-order' s, the ratio of the lengths of spectral lines at various orders no longer meet the relation of normal gratings.