In this paper, we derive the response function of angle signal in a two-dimensional X-ray grating interferometry system under the condition of parallel coherent light, and depict the surface of the function with Matlab.Although there are four kinds of commonly used beam splitter gratings and three kinds of analyzer gratings, and there are still different compound modes between them, we may find that the ultimate surface of the response function of angle signal can only be of three kinds: the peak type, the valley type, and the peak-valley symmetry type of shifting surfaces. As there is a numerical complementary ralationship between the peak type and the valley type of shifting surfaces, we can take the two kinds as one; and finally we only need to consider two kinds of shifting surface.This conclusion simplies the common understanding of the two-dimensional X-ray grating interferometry method, and lays the foundation for the research of quantitatively extracting the two-demensional signal in the future.