Laser combining is the main way to develop the high power diode laser source. Traditional beam combining is used to increase the power but leads to a poor beam quality. Spectral beam combining with external cavity feedback improves the characteristics of the power and the beam quality by the way of combining diode laser oscillation with external optical system. By spectral beam combining with external cavity feedback based on a transmission grating, a diode laser source is developed with a continuous wave power of 140.6 W, a beam quality of 4.367 mm·mrad and a spectrum width of 36.60 nm. The whole spectrum is composed of 56 independent wavelength peaks, each one of which is only corresponding to one of the combining laser units, respectively. A feasible solution to obtain a good beam quality for high power diode laser source is demonstrated.