Abstract: The theoretical model of spectral beam combining of fiber lasers is presented. It includes a fiber laser array, a lens and a diffraction grating. Based on the conversion and diffraction theories of Gaussian beam, far-field optical intensity expression is derived, and the systemic parameters are optimized. Numerical results show that diffraction efficiency can be improved by overlapping the negative first-order interference principal maximum and the zeroth-order diffraction maximum of the central-wavelength-laser, overlapping the zeroth-order interference principal maximum and the first-order diffraction minima of the central-wavelength-laser, and overlapping the negative first-order interference principal maximums of other lasers and the zeroth-order diffraction maximum of the central-wave