Abstract: Qualitative and semi - quantitative analysis on the ultrashort pulse laser and optical frequency comb is made using the wave interference theory and Fourier transformation. Based on the known conclusions about the multiple beam interference，we analyze the principle of mode locking technology，the temporal characters of pulse laser. Using the Fourier transformation and Fourier series，we demonstrate the physical image of the optical frequency comb，derive the relation between the pulse length and the spectral bandwidth，the relation between the temporal pulse distance and the spectral frequency interval.

Key words: ultra fast laser, femtosecond laser, frequency comb, mode - locked, interference