Abstract: In order to generate shorter wavelength FEL, and electronic bunch has to be coupled with the extrnal powerful laser pulses in optical klystron. In this process the coherent harmonic radiation will be generated. It is necessary that the electronic bunch and laser pulses are synchronic in both time and space. The synchronization in space can be realized by adjusting the light road of the external laser. In this paper, a method is suggested to obtain the synchronization in time.