Aiming at the problem that there is no comprehensive model of coherent beam combining through superposed volume gratings,we establish a comprehensive model for two-beams combining and its analytic formula is derived using coupled wave theory.It shows that the intensity of combined beam is influenced by many factors such as the amplitudes and phases of the two incident beams,the phase shift between gratings,the amplitude of modulated refraction index,the thickness of gratings and the detuning et al.The effect of each optical parameter on combined beam is numerically analyzed in detail.And the sufficient and necessary conditions to realize efficient beam combination were summarized,such as the thickness of gratings must be optimized,the index modulation of each grating must be equal and the Bragg mismatching must be small enough et al.