Aiming at the application requirements of spectral calibration at 2.04 μm channel of the spatial heterodyne spectrometer used for spaceborne CO2 remote sensing detection, a narrow-linewidth, tunable and highly-stable Tm/Ho co-doped fiber laser (THDFL) is developed. The effective longitudinal mode spacing is expanded by using Wiener effect of the linear compound resonant cavity. The multi-mode laser oscillation is effectively suppressed, and the output laser linewidth reaches 7.5 MHz. By axially stretching the FBG packaged with temperature compensation and vibration isolation techniques, the THDFL can be tuned from 2040 nm to 2042.5 nm, and the output wavelength uncertainty is less than 5×10－3 nm. The output power of THDFL reaches 192 mW with the developed 1565 nm fiber laser pumping source. Using the output laser provided by the tunable THDFL, spectral calibration of the spatial heterodyne spectrometer at 2.04 μm channel is realized after being originated to a wavelength standard, and the calibration accuracy is better than the set index requirements.