A spectral beam combination (SBC) laser with dual multi-layer dielectric (MLD) grating dispersion compensation configuration is studied theoretically and experimentally, which can combine multiple-channel fiber lasers into a common aperture beam with high beam quality and reduce the linewidth requirement of the single channel fiber laser. The analysis model of the degradation of beam quality is optimized. The influences of the laser wavelength, the dispersion of grating and the spectral structure on the output beam quality of the SBC laser are analyzed. The variation of the beam quality of the SBC system at different output power levels is also studied experimentally. The output combined beam with the maximum average output power of 9.6 kW is obtained, of which the beam quality factor M2 is 2.9 and the combination efficiency is 92.0%. By reducing the linewidth and improving the output power of each channel fiber laser or increasing the number of channels, the common aperture SBC output beam can be further scaled with higher output power and higher beam quality.