Abstract: Grating spectrometer is one of the most important instruments in studying solar radiation, and accuracy of its wavelength scanning mechanism determines final measurement result. Taking errors from both grating parameters and machining dimensions into consideration, this paper analyzes comprehensive accuracy of wavelength scanning mechanism which is made up of screw and swing. Based on dispersive principle of concave grating, relationships between wavelength λ and displacement of end swing along direction of screw x, swing length l, grating constant g and half angle between incident light and diffraction light δ are derived. Then derivatives of equation are obtain, and wavelength errors are calculated between 650 nm and 2 400 nm and should be no more than ±1.227 nm according to principle of errors stack. Then results are tested on prototype. Fitted curve of wavelength and error can be obtained when taking mercury lamp as light source, and can be checkeded when taking helium neon laser as light source. Results prove correctness of theorectical calculation method, and provide theory evidence in choosing components.