According to Fresnel-Kirchhoff diffraction formula, the light intensity expression about diffraction at rectangular apertures was obtained and numerical calculation was carried out, influence of the size of aperture on light intensity of rectangular diffraction was analyzed. The light intensity distribution was characterized by Fraunhofer diffraction when the projection of light source is located in the symmetry center of rectangular aperture and the size of aperture is small enough. With increasing of the size of aperture, the ratio of adjacent peak and valley value of light intensity decreases in the same region of geometric shadow, and more light intensity peaks appear in the geometric illuminated district. The symmetry no longer exists when the projection of light source is not located in the symmetry center of rectangular aperture. At last the light intensity distribution of Fresnel diffraction at several rectangular apertures is calculated and analyzed.