As a kind of spectroscopic instrument, the monochromator has important applications in radiation calibration of sensors. In the actual application process, the setting of wavelength and bandwidth has an important influence on the precise calibration of the sensor.The low-pressure mercury lamp is used as the wavelength standard light source. Through the method of characteristic line scanning, it is studied that in the process of sensor calibration, it has an important influence on the precise calibration of the sensor under different slit widths of the monochromator. The results show that when the exit pupil is the same and the width is changed at the same time, the maximum deviation of the wavelength is 0.17 nm compared with the typical calibration state (the exit slit width is set to 0.5 mm); when the incident slit is inconsistent with the exit slit, the maximum wavelength deviation is 0.18 nm compared to the typical calibration state; when the light source is not filled with the entrance slit and the full aperture is filled, the maximum error is 0.02 nm, and the effect is almost negligible. In the related experimental study of sensor spectral radiometric calibration, the calibration accuracy and accuracy evaluation of monochromator have important applications.