In order to satisfy the technical requirement of soft X-ray microscopy beamline in Shanghai Synchrotron Radiation Facility (SSRF), whose key assembly monochromator is designed. Wavelength scanning movement principle of monochromator is described. Design scheme of wavelength scanning mechanism is discussed, and factors affecting the angular repeatability of plane mirror and plane grating are analyzed in detail; switching mechanism of plane grating is described, and horizontal deviation, vertical deviation, roll angle precision, yaw angle precision and pitch angle precision are analyzed in detail; six-bar parallel mechanism is used for adjusting the UHV-chamber, and adjusting range and resolution of the bar are analyzed. The entire structure of monochromator is presented, and its precision testing is performed. Results show that the angular repeatability of plane mirror and plane grating are 0.166″ and 0.149″, and roll, yaw and pitch angular repeatability of plane grating switching mechanism are 0.08″, 0.12″ and 0.05″, indicating that structure design and precision of monochromator satisfy the technical demand.