Aiming at the problem of beam quality evaluation for external cavity-based spectral beam combining system with microlens array, the theoretical model for evaluating the beam quality of beam-combined laser is established. According to the model, the influences of relevant system parameters on beam quality are numerically analyzed in detail. The simulation results show that the beam quality is mainly influenced by the defocus value, the focal length of microlens and the mode field radius, while the influences of the array width are little. With the array width changing from zero to 10mm, the beam quality factors in x and y directions are preserved near 1.225 and 1 respectively. Compared with the traditional spectral beam combining system, the incorporation of microlens array effectively improves the beam quality of output laser. In order to keep the beam quality factor in x direction smaller than 1.3, the defocus value ought to be restricted in a range of -0.017~0.017 mm. For spectral beam combining system with large mode area double-clad fibers, the beam quality can be improved through utilizing microlens array with longer focal length.