A long-wave infrared (LWIR) on-chip gas sensor based on subwavelength grating waveguide is proposed. By optimizing the grating structural parameters, the corresponding slow-light region is overlapped with the absorption spectrum of methane, which can greatly improve the light–gas interaction to achieve excellent sensing performance. The presented waveguide gas sensor is designed to operate at the wavelength of 7.70 μm, which corresponds to the methane absorption peak in the LWIR and exhibits a high slow-light enhancement factor of 7.514. The related sensitivity and limit of detection are, respectively, 26.54393 and 0.1327 ppm.