It is an effective approach to obtain high-power laser output by spectral beam combining (SBC) technologies. In this paper, a simple SBC system based on the superimposed reflective volume Bragg grating (RVBG) is proposed to reduce the scale of the SBC system. Considering the cross-coupling between the two gratings in a volume, two structure models of the superimposed RVBG-the superimposed RVBG with the same period and the superimposed RVBG with a different period-are analyzed and compared by using the rigorous coupled wave analysis, and their applications for SBC system are discussed. Numerical results show that the superimposed RVBG is easy to be fabricated and has the potential to combine multiple lasers with one volume. Cross-coupling can be neglected for the same period structure when the slant angle divergence is larger than 0.01 rad and for the different period structure when the spectral separation is larger than 1.0 nm. The superimposed RVBG with a different period can achieve high diffraction efficiency simultaneously for each grating with a small slant angle divergence, and is convenient to combine multiple beams with small spectral separation. The effect of wavelength shift on diffraction efficiency can be ignored.