Abstract: Monochromator is a spectroscopic instrument, which decomposes a polychromatic light into monochromatic light with different wavelengths using spectral dispersive element. This paper presents a new calibration method for three-grating monochromator. For the monochromator adopting sinusoidal scanning, its accuracy depends on the number of calibration points. This paper uses sine curve fitting method according to the design of three-grating scanning monochromator. Since grating equation is in compliance with the law of sine, a small number of calibration points could meet the requirements of fitting accuracy. Appropriate parameters to be determined are selected, which significantly reduces the computational complexity in the process of fitting. The analysis results of simulation software show that the wavelength fitting error is less than 0.5 nm.