The propagation model for the spectral combining systems including Yb-doped fiber laser arrays，transform lens，grating and output coupler has been built up. By taking into considerotion the grating angle dispersion，the fabrication errors of groove angle and the diffraction efficiency of the grating，the formula for the phase change of the Gaussian beam unparallelly oblique incidence on the grating and the analytical expression for the intensity distribution of the combining beam have been derived by using the ray tracing method，the diffraction integral method，the principle of incoherent superposition of laser beams and the intensity second-order moments method. Consequently，the effects of the angle dispersion，the diffraction efficiency and the fabrication errors of groove angle of the grating on the beam characteristics of the spectral combining system have been analyzed for the case of Gaussian beam unparallelly oblique incidence on the grating. The results show that the beam quality of the combined beam remains almost the same as that of a single fiber laser. The effect of the angle dispersion of the grating on the beam quality of combined beam can be neglected. The beam quality of combined beam degrades with the increasing of the fabrication errors of groove angle of the grating. For the case of the large fabrication errors of groove angle of the grating，the effect of the diffraction efficiency of the grating on the beam characteristics of the combined beam should be considered.