The development of ultraviolet and soft X-ray monochromators with high performances is very important to the applications of high-brightness synchrotron radiation light source. The varied line-space plane grating monochromator by Itou is studed in detail, and it is found that its grating working curve is nearly independent of the monochromator parameters, such as entrance-arm and exit-arm lengths, and only determined by its boundary condition. The curve equation is similar to that of the SX-700's. The monochromator has both very good grating-diffraction-efficiency and high-order-suppression performances. Moreover, it is very convenient to optimize the monochromator performances and solve the problem of space conflict caused by more than one beamlines that share the same storage-ring window.