Based on the dual-Doppler radar data of the mobile weather radar (CINRAD/CCJ) and Changle weather radar (CINRAD/SA), the characteristics of three-dimensional wind structure in the puissant and weakening phases of the strong banded echo of super typhoon Sepat (0709) on 18 August 2007 are examined by means of dual-Doppler radar retrieval in the Earth coordinate. The results reveal that: 1) During the puissant phase, there were strong east by north wind centers (strong wind core ) along the right or the back of the right side of strong echoes in their course, even weak wind center existed in the front of the strongest echo. The structure maintained from the lower layer to the mid-layer, thus the strong echoes corresponded to the horizontal convergence and positive vorticity, resulting in significantly upward movement which helped the development and maintenance of convection. And the clouds migrated quickly. Relative to the moving clouds, the anterior and posterior air in the mid－lower layer flowed to the strong echoes, and then converged and ascended with maximum vertical velocity between strong echo center and the wind core on its northern side. In the meanwhile, divergence appeared in the higher layer over the strong echo area, and the air mainly flowed out backward. 2) During the weakening period, the weak wind center was located at the strong echo center or its north side with negative vorticity. The clouds moved slowly and the easterlies went through the clouds. The convergence over the echo area weakened and the obvious updraft appeared in the mid-layer strong echo near the typhoon center. Thus the banded echo has been difficult to maintain. 3) Strong wind core could transport the momentum and water vapor of weakening spiral cloud bands on the posterior side to the developing area of the banded echo; therefore the strong wind core structure was likely to be the main cause for the rapid development of banded echo.