Abstract: Because of the distributed transmitting features, it is difficult to test the radiation power-aperture of active phased array radar directly. Firstly, a new test principle and method of power-aperture product is provided. Secondly, according to the engineering reality, the direct test method and comparison test method are both provided, and the test results are compared. Thirdly, the alignment algorithm of the normal direction of wave-beam and the analysis of testing error are provided. Finally, the correctness and validity of the algorithm is indicated by the engineering application of testing power-aperture product of one active phased array radar.