In order to improve the speed and accuracy of spectral calibration for echelle spectrometers, an automatic spectral calibration method by multiple characteristic wavelengths of mercury lamp is designed based on the spectrum retrieval algorithm. Spectral calibration experiments are conducted with a mercury lamp as the calibration light source. The results show that the spectral retrieval model is automatically corrected as long as the algorithm is within the range of the spectrum deviation. Choosing more calibration wavelengths and more uniformly distributed calibration wavelengths results in higher calibration accuracy. For the echelle spectrometer with wavelength range from 250 nm to 600 nm, choosing more than five calibration wavelengths will make the calibration accuracy be 0.01 nm, which is the theoretical resolution of the spectrometer. The automatic spectral calibration method makes echelle spectrometers more practical and valuable in engineering applications.