The atmospheric temperature is an important meteorological parameter. The rotational Raman lidar temperature measurement system is usually used for retrieval of atmospheric temperature according to the echo signal intensity ratio. UV double grating monochromator (DGM) is a kind of spectroscopical equipment for extracting the rotational Raman's spectra. The UV DGM consists of optical fibers , focal plates , lenses and gratings. Correct parameters of the gratings, lenses, fibers are essential to the successful development of DGM. First, a suitable grating is selected according to the grating diffraction theory. Second, the diameter and focal length of lens are chosen according to ruled area and blaze angle of grating, the numerical aperture of fiber and how difficult machining is . Finally, the range of fiber core diameter is chosen according to the distance between the sixth and the twelfth spectral line on the focal plane. In order to compare the different fiber core diameter's statistical temperature error, and suppression ratio of elastic scattering light, some numerical calculations are conducted. The simulation results show that the measurement accuracy increases in the evening and approximately decreases during daytime with the increase of input fiber core diameter .As the input fiber core diameter increases, the rejection ratio is decreasing ,but they are all more than 7 orders of magnitude. It is preferable to choose input fiber with core diameter of 300um during the daytime and 600um at night. The correct parameters of grating, lens and fiber provide theoretical basis for the development of the UV DGM.