Based on the theory of light propagation and diffraction characteristics of grating, the optical energy transfer of bi-grating imaging system is studied and the expression for the intensity of combined beams is given. Combined with bi-grating imaging formula, the influence of bi-grating properties and imaging place on intensity of combined beams is analyzed. The results show that intensity of combined beams is closely related with diffraction efficiency and location of two gratings. In three bi-grating imaging systems, the intensity of combined beams is measured. The experimental results are consistent with theoretical anticipation.