The condition of spectral beam combining,which uses transmitting volume Bragg grating for two laser beams with different wavelengths,was analyzed. Based on coupled-wave theory,the diffraction efficiency equation of Bragg grating was derived. An optimal design requirement of spectral beam combining was obtained,and an approach to control the beam incident angle by using a convex lens was proposed. On the basis of the optimal deaign,spectral beam combining was modeled, and the effect of grating parameter on diffraction efficiency was analyzed. It shows that different grating parameters must be selected for different grating thickness,and spectral selectivity increases gradually with decrease of grating thickness at same refractive-index modulation and spatial frequency. This study further shows that grating diffraction efficiency decreases grad-ually with increase of beam divergence.