The double grating monochromator for a pure rotational Raman-lidar can provide for better than 10-7 suppression of the spectral line due to unshifted Mie+Rayleigh scattering, obtain high-purity rotational Raman spectrum, and improve transmission and stability of the optical system. The complexly structure of the double grating monochromator is always difficulty to Raman-lidar. Two optical paths of grating are compared and a conclusion can be drawn that the structure of beam path of grating of symmetrical arrange of optical fibre is better in grating efficiency than the structure of beam path of grating of linear arrange of optical fiber. Then a formula about relative of range of diffractive angle and order of grating is obtained through grating equation and rotational Raman equation. When grating constant is 600 line/mm, the fifth order of grating is optimal.