Based on the diffraction theory of monochromatic scalar wave，the characteristics of plane wave with uniform intensity diffracted by aperture-array from different angles were investigated. Considering the effect of the near apertures, the theoretical model and intensity distribution numerical integral expression were established on the base of aperture diffraction theory, where the apertures are hard-edged. The intensity distribution patterns of diffraction when plane light with 500 nm wavelength went through the aperture-array were simulated with Matlab, and figures of one- and two-dimensional diffraction intensity distribution of aperture and apertures-array. The simulation results were applied to design optical system's structure parameters of a micro digital sun sensor and to determine the parameters in image processing of it. The experiments showed that simulation results of intensity distribution diffraction patterns were correct and the design results of sun sensor's optical system were resonable. The aperture-array diffraction theory provided a reliable theory for the optical system design of the sun sensor and image processing.