In response to the lack of methods for calculating the aperture averaging function of optical receivers with arbitrary shapes in existing research, a novel two-dimensional mapping method is proposed. Firstly, the mathematical shape of the receiver is mapped into a two-dimensional matrix, and based on this matrix, a reference aperture and a displacement aperture are constructed, and the displacement range is determined. Then, a series of variable displacement matrices are obtained by gradually moving the displacement aperture. Subsequently, by calculating the same elements between the displacement matrix and the reference matrix, the aperture overlap area under different centroid spacing vectors is obtained, thereby accurately calculating the aperture averaging function of the receiver. Finally, taking the calculation of the aperture averaging function of a Cassegrain telescope as an example, it is mapped into a 300×300 matrix and compared with the theoretical calculation results. The comparison results show that the method has extremely small errors in calculating the aperture averaging function, verifying the accuracy and reliability of the proposed method.