Abstract: Analysis for the radiation pattern of antennas with physical optics method is now widely used in the calculation for far field pattern of reflector antennas, which has fast computing speed and low memory requirements. The expression to calculate the far field of reflector antennas using the Stratton-Chu integral formula is derived based on surface current, and the directivity of an offset reflector antenna calculated in MATLAB is right using this method comparing with the results in the reference. The expression to calculate the gain of antennas is derived with cos^qθ as pattern of the feed. The offset reflector antenna is shaped for good gain flatness during scanning in the pitching direction for 20°, and the far field pattern of shaped reflected antenna is analyzed. Results of gain of the reflector antenna in different scanning angles before and after shaping is calculated, which show that the gain flatness is improved by shaping and less time and lower memory space is taken using this method to analyse the far field pattern of antennas with physical optics method.