To achieve the wavelength repeatability ±0.05 nm of the infrared double grating spectrometer, the wavelength scanning mechanism of the spectrometer is designed and the error source that influences the wavelength repeatability is analyzed. According to the structure characteristics and indicator requirements of the spectrometer, the sine mechanism based on lead screw and rocker is designed as the wavelength scanning mechanism. On the basis of concave grating scanning principle, formula from optical parameters to mechanism parameters is derived and the influences of each error source on the wavelength scanning mechanism are analyzed. The analysis results show that the repeatability of the spectrometer in the range of 760~2200 nm should be better than ±0.05 nm when the repeated positioning accuracy of the lead screw is ±1.2 μm. Infrared double grating spectrometer prototype is designed, and experimentally verified. Using mercury lamp as light source, the wavelength repeatability of the wavelength scanning mechanism at the characteristic wavelength is calculated after scanning each characteristic wavelength for 7 times. The experimental results show that the wavelength repeatability of the spectrometer is in the range of -0.038~0.041 nm, and the wavelength scanning mechanism can meet the requirements of the application.