Abstract: method of CAT for radiation performance test on space borne communications system level has been developed. Compared with the conventional method, by which the system performance data are got by calculation of the data gained in subsystems tests, the method described here has got higher accuracy. The test and software was set-up with a focus on getting directly the most important system charactritics,that the users are most interested in,such as EIRP coverage,coverage rale of service zone, G/T and SFD coverage.A test on the payload of DFH-3, the third generation of Chinese communications satellite, was performed and the results of the test were satisfactory.