Analysis of the optical characteristic on the filtering plane of the projection system based on grating light modulators (GLM) illuminated by extended light source is the key section of the optical information processing. The optical characteristic on the filtering plane of the projection system illuminated by a laser facular of certain size is analyzed using the theory of partially coherent light and the numerical simulation. The result shows that the diffraction light intensity distribution on the filtering plane is the convolution of the squared Fourier transform of the modulator′s transmittance function and the geometric projection of the laser facular size on the filtering plane. The diffraction order extends on the filtering plane,and the aliasing phenomenon appears that through the distance between the filtering plane and the modulator increasing while the degree of aliasing decreases. The relevant experimental system is constructed and the diffraction pattern on the filtering plane is obtained. The experimental results accord with the theoretical analysis. In the end,we propose that a converging lens set between the modulator and the filtering plane could reduce the optical dimension of the system,and a method for improving the effect of system information processing.