The accurate optical cross section areas (OCSA) calculation of spatial objects is an important foundation and prerequisite for the analysis and recognition of spatial object characteristics. Aiming at the problems of the poor real-time performance of the grid model used in the faceting method and the weak ability for the description of bidirectional-reflectance-distribution-function (BRDF) of material by using the computer graphics method, an OCSA calculation method for complex space objects based on the OpenGL picking technique is proposed. The primary occlusion judgment of facets is realized by the OpenGL picking technique, and the second occlusion judgment between facets is realized based on the improved Z buffering technique. Therefore, the detailed information of facets is preserved without loss of real-time performance, which makes the application of precise BRDF model and the accurate calculation of OCSA possible. One nested cylinder and one satellite model are designed, and their OCSA value is calculated. The maximum OCSA error of the nested cylinder is less than 0.08% and the average time consumption on a general computer is less than 0.01 s. The average time consumption of OCSA for a satellite is less than 0.1 s. The results verify the correctness and real-time performance of the proposed method.