For binary optical elements, diffraction efficiency is one of the important performance indexes. Under the oblique incidence cases, the optical path difference of light beam distributed on the edge of steps can be calculated by using the geometric method. Then, according to the multi-slit Fraunhofer diffraction, the expression of diffraction intensity of reflection step grating is deduced. Under the two limit conditions that the number N of different steps changes from 2 to infinity, we discuss the expression of light intensity’s change. In general case, we deduce the expression of diffraction efficiency which is different from that of other articles. Experimental results show that the step number and ratio between the etching depth and the light wavelength are closely associated with the diffraction efficiency of reflection step grating.