The design principle of a grazing incident multi-included-angle plane grating monochromator is introduced, and the feasibility of the monochromator is analyzed and verified theoretically. This system is an improvement of SX-700 monochromator. Rising or falling the multi-angle plane reflection mirror can easily change the included-angle of covering a certain wavelength range, thus the desired wavelength of monochromatic light is chosen. Also, the monochromatization effect with X-LAB optical simulation software by monochromator at soft X-ray region 1-2.48 nm are simulated when the light incidents in the reflecting surface of a certain angular in multi-angle plane reflection mirror. This proves in theory that the wide energy spectrum scanning and high harmonics suppression can be realized with the proposed design. Furthermore, the structure of the monochromator is simple, which makes it easy to popularize for practical applications.