In Diode Laser Array (DLA) spectral beam combining (SBC) systems with grating-external cavity, the beam quality of the combined beam tends to degrade due to comprehensive function of factors involving divergence angle of light, positional deviation and direction angle deviation caused by “smile” effect of emitters, etc.. Based on the consideration of the effects of the divergence angle and “smile” effect of DLA emitters on the beam propagation characteristics, the light propagation model of DLA SBC systems with grating-external cavity has been built up, and the effects of divergence angle and the positional deviation and direction angle deviation caused by “smile” effect of DLA emitters on the beam quality of the combined beam in SBC systems have been analyzed quantitatively. The results indicate that, the beam quality of the combined beam would be significantly affected by the DLA light source quality, i.e., the bigger the divergence angle and the direction angle deviation caused by “smile” effect are, the worse the beam quality of the combined beam is. The positional deviation of emitters in beam-combination direction caused by “smile” effect has little impact on the beam quality of the combined beam, whereas the positional deviation in non beam-combination direction would significantly degrade the beam quality of the combined beam. In practical applications, measures should be taken to improve the DLA light source quality in order to eliminate the effect on the beam quality of the combined beam.