Using the well-known KMR equations and combined with BFEL's overall experimental parameters, this paper presents a study of Optical Klystron (OK) configuration for the Beijing Free Electron Laser (BFEL) which will start to operate recently. The effects of the laser input power, electron beam energy distribution and its current, dispersion magnetic field, drift region distance and its position in the wiggler, upon the Free Electron Laser gain, are analyzed in detail. Based on the oscillator configuration of BFEL, a group of practial optical klystron parameters with a suitable beam energy spread has been put forward and the analyses of its saturation power, power spectrum, and tapered wiggler also have been made.