We studied the spectral beam combining (SBC) of a large optical cavity (LOC) laser array to achieve high-power and high-brightness laser output. We discussed the characteristics of the external cavity feedback efficiency and the focal length of the transform lens for lasers with different waveguide thicknesses. We have found that using LOC laser diodes can increase the proportion of external cavity feedback, thereby improving the SBC efficiency. At a current of 90 A, the CW output power of the SBC system is 59.2 W, and the SBC efficiency reaches up to 102.8%. All emitters of the laser array have achieved spectral locking with a spectral width of 11.67 nm, and the beam parameter product is 4.38 mm·mrad.