In order to improve the capability of forecasting and warning of rainstormcaus ing MCC, the diagnostic analysis was made on the synoptic cause and the 3dimen sional flow struc ture of MCC on 2 July 2006 on the Middle Yellow River by using the satellite ima ge, MICAPS and NCEP and Doppler radar data. The result shows that the MCC was t he direct influence system, the formation of warm and wet shear convergence in t he middlelow level, the αscale divergence in the higher level, and the lifting force from low level caused by the cold air mass frontogenesis, all thes e circulation background are favorable to the MCC. The MCC occurred on high ener gy and weak convective instability zone, where the southwest jet on 700hPa and s outh flow on 850 hPa provide plenty of water vapor and energy for MCC. The coupl ing of divergence field and vorticity field and the formation of updraft is the driving mechanism of MCC. The velocity images of Doppler radar show that the low southeast jet develops and maintains, the low jet to flow into the convergence line maintains and the bowlike αscale divergence belt on middle high le vel develops and maintains, all these constitute the 3dimensional flow structu re of the MCC.