Optical design of a novel spherical grating monochromator is described, which is used in testing the soft X ray spectra of a low debris laser produced plasma (LPP). By employing a variable deviation angle at the grating, it is possible to exactly focus the entrance slit on the exit slit in the wavelength range of 3 nm～20 nm. The effect of the key parameters is analyzed, and the optimum design is implemented. A resolution of λ/ Δ λ≥2000 at 13 nm has been achieved (the entrance slit is 50 μm).