An algorithm of plots-centroid based on 3D phased-array radars scanning and echo data structure is proposed. According to the image structure of target echo data, the radar operating parameter and the scattering area characteristic of the target, using the classical image processing flooding algorithm and center of the mass algorithm to achieve the target plots centroid and detection output in 3D dimensions of distance, azimuth and pitch. The algorithm can ensure the dynamic and real-time output of the plots, and reduce the delay of the output of the plots. It has great practical value in engineering.