In spectral beam combining systems, the locked spectra of the diode laser array are determined by the interaction between the internal cavity and the external cavity. Using the multibeam interference theory, spectral modes locked by the coupled cavity are numerically calculated and analyzed. A spectral mode of a diode emitter is divided into a main lobe, a left sidelobe, and a right sidelobe, which is mainly affected by the position of the diode emitter. Based on the locked spectral modes, a beam propagation model in the grating-external cavity is established by the diffraction integral method and the incoherent superposition principle. The effects of the interval between two adjacent emitters, the reflectivity of the front surface, and the distance between the grating and the coupler on the characteristics of the combined beam have been discussed in detail. The results show that the beam quality and the spot size of the combined beam gradually degrade with the increasing interval. By reducing the reflectivity of the front surface of the diode emitter and decreasing the distance between the grating and the coupler, the beam quality can be improved. Among the three factors, such as the interval between two adjacent emitters, the reflectivity of the front surface of an emitter, and the diffraction distance, the interval has the most sensitive impact on the combined beam characteristics. Thus a beam shrinker to convert big beams into small beams in diameter can be adopted to optimize the combined beam.