The multilayer grating combined structure can be obtained by depositing the soft X ray multilayer on the laminar grating. In this paper, we report the diffraction performance of the multilayer grating under the irradiation of Cu K α X ray at low angle. Analysing with X ray kinematics theory we obtained two diffraction conditions, corresponding to the grating equation and Bragg law respectively. The efficiencies of grating were found to be modulated by the multilayer reflection. The values both of experiment and theory of diffraction peak positions and intensities are compared and analyzed under three different scan methods such as specular reflection scan, sample scan and detector scan. The results show that they are coincided well.