Assuming that the incident light is from different directions, the process of the incident light passing a series of grids with different intervals has been described and analyzed by applying two-dimensional Fourier optical method. A formula for the distribution of the collimated beam's spatial frequency is established and the spatial frequency distribution of the incident light is obtained. The main procedures and key points in the calculation are listed. When the incident light is from every direction, the calculation demonstrates the collimation performance, when the incident radiation strikes on the collimator paralle, the calculated result shows the diffraction of the collimated beam. The diffraction reaches out of the geometrical shade. The two-dimensional distribution of the collimated beam's spatial frequencies is obtained. The data agree better with the published experimental result than the one-dimensional calculation of foreign peers. This method is a quick and accurate way to calculate the performance of the collimator.