A propagation model for spectral beam combining is set up in this paper. First, using ray tracing method, the phase change induced by the blazed grating is expressed with the optical path differences in the groove and between grooves and the calculation model for the phase change of the beam obliquely incident on the grating is constructed. Then, using the diffraction integral method and the principle of incoherent superposition, the analytical expression for the output field of each emitter after passing through the spectral beam combining system is given and the intensity distribution of combined beam is calculated. Finally, using to the intensity second-order moments method, the M2-factors of the array beam and the combined beam are calculated and the effect of the system parameters on the characteristics of the combined beam is analyzed quantitatively. The results show that the M2-factor of the combined beam after passing through the spectral beam combining system remains almost the same as that of an individual emitter. The spacing of center wavelength between each emitter decreases with the increasing focal length of the transforming lens, resulting in the narrower spectrum width of the combined beam. The encapsulation error of the array elements has relatively little effect on the intensity distribution of the combined beam, whereas the position error of the array source has relatively greater effect on the combined beam.