Lijiang Exoplanet Tracker (LijET) was designed to detect exoplanets with extremely high precision radial velocity (RV) measurements, and it was mounted on 2.4m telescope at Lijiang Observatory in 2011. The Dispersed Fixed- Delay Interferometry (DFDI) mode of LiJET is a combination of a thermally compensated monolithic michelson interferometer and a cross-dispersed echelle spectrograph. When the slit width is 1.6”, the spectral resolution is 18000. With a 4k x 4k CCD, the spectrograph has wavelength coverage of 390nm-690nm. The temperature stability of the instrument is 25±0.001°C, and the pressure stability of the instrument is 10.9±0.001psi. LiJET realize high precision RV measurements by measuring the phase shifts of fringes in the slit direction. Differential RV is a function of light speed, phase shift, wavelength and optical delay. Thus, optical delay is necessary to be determined accurately to take differential RV measurements to derive precise RV. We used thorium argon (ThAr) and tungsten lamp to calibration the DFDI spectrum of LiJET, and then to calculate the optical delay at different channels on the CCD detector.