A theoretical method to obtain the soft X-ray multilayer monochromater of-better energy resolution is put forward. The basic point is to choose the alternating elements with new criterions instead of general choosing method of high-and low z elements. Moreover, a simple and practicable multilayer designing method to achieve high reflectivity was presented. The some results of soft X-ray multilayer monochromater study in recent years on Beijing Synchrotron Radiation Facility (BSRF) have been given out.