High spectral resolution is the inevitable trend in the future development of spectral imager. However, the traditional spectral calibration based on the monochromator in order to ensure enough output power made bandwidth of the calibration light source and spectral resolution of the spectral imager become very close, which would significantly affect the precision of the calibration. This paper quantitatively analyzed that the bandwidth of calibration light source bring the influence on the spectral calibration, and the methods of deconvolution or pixel merging was put forward to correct the calibration error. Finally, calibrated the spectral imager with different bandwidth light sources, and corrected by the before-mentioned methods. The results show that the accuracy of the spectral resolution is better than 0.2 nm, achieves the accuracy of the spectral calibration system and verifies the feasibility of the method.