The frequency-domain optical delay line(FD-ODL)is a kind of optical scanning technology,which consists of a diffraction grating,a lens,and a scanning mirror. When employing FD-ODL in optical coherence tomography(OCT)system,the reference scanning speed at meter-per-second can be achieved to support image acquisition at video rate. An analysis about the principle of the FD-ODL is made at the point of geometry optics. The results show that the frequency of the optic length or phase variation is almost linear to the scanning angular frequency as the scan angle is very small. The image of an OCT system based on this structure proves that it can avoid blur induced by the displacement of the living tissue. In addition,a new scanning device is also proposed.