The angel between signal light and local light is an important factor to the heterodyne detection. When the angle is too large, one even cannot detect the heterodyne signal. It is proposed that the amplitude of heterodyne signal output by detection is just the Fourier transform of aperture transmittance function. Based on the theory, a method that can reduce the influence of spatial mismatch angle on heterodyne detection is proposed. Namely, if a component whose transmittance is a certain function is placed on the aperture surface and the Fourier transform of the function meets certain requirement, one can find that the amplitude of heterodyne signal may be enhanced when the angle between the two beams becomes large, which can reduce the requirement for pointing accuracy of detection system.