Abstract: Multi scale analysis of an excessive precipitation process occurred in the southern Shandong from August 17 to 18 in 2009 is performed using various observation data and reanalysis data with resolution 1°×1° from NCEP. The results indicate that this process is caused by the interaction of westerly trough at 500 hPa, warm shear line at 850 hPa and inverted trough in ground layer. Before the excessive precipitation occurred, there are deep moist layer and weak vapor convergence in the mid- and low-level. The atmospheric stratification shows convective instability with more unstable energy. The convergence of warm shear line and warm advection in the low-level together with the positive vorticity advection, side convergence and development of slantwise vorticity in the mid-and upper-level make the vertical vorticity increased and ascending motion developed. The southeast airflow in the low-level coordinating with the upper trough generate a secondary circulation whose vertical upward branch makes ascending movement strengthened triggering convective unstable energy release to generate severe convection, and then form severe precipitation. During the severe precipitation, intruding of weak cold air in the mid-and upper-level makes convection strengthened and precipitation reinforced. Mesoscale convective clusters originate in east part of inverted trough with low pressure and near the mesoscale convergence line in ground layer. The inhomogeneous heating and cooling on the ground lead to enhance the small scale temperature gradient of the inverted trough with low presure, and the center of excessive precipitation emerges in the temperature gradient zone. The severe precipitation is generated by both mesoscale convective systems with long strip shape and round-shape mesoscale convective cloud clusters developed in the north tip of the above-mentioned convective system. Both mesoscale convective systems and mesoscale convective cloud clusters move slowly from west to east showing belt-shaped strong echoes, and in the radar echo intensity chart it is appears that the echo belt of mesoscale convection is converged continually in a anticlockwise direction in the north moving process.