Aimed to maintain excellent beam quality, the influence of pointing deviation on the beam quality is theoretically studied in the dual-grating spectral beam combination (SBC). The incident light field of the fiber laser array with the pointing deviation is built by the transformation of coordinates, and the variation rule of the combined beam quality with random perturbations is discussed by the principle of beam diffraction and the statistical analysis. As a result, the degradation of beam quality for the pointing deviation is respectively 0.31(±0.13) and 3.06(±1.27) for the standard deviation of 0.1 mrad and 0.5 mrad, spreading as a Normal distribution. It can be concluded that the pointing deviation of laser array will destroy the condition of the SBC of the common aperture output, resulting in the continuous growth of the M<sup>2</sup> factor. These analyses provide a valid basis for setting up the experimental system of dual-grating SBC.