With observational data, mesoscale automatic weather station data and NCEP reanalysis data, a diagnostic analysis is made of the causes of a severe heavy rain occurred on 18 July 2007 in Shandong Province. The results show that the rainstorm was caused by the interaction of the westly trough originated from the cold vortex over the northeast China, the quasi eastwest oriented shear line at lower levels, and the warm and wet airflow on the northwest edge of the subtropical high and cold air from northeast. It is found that the continued warming and strong warmwet air transport at lower levels provided excellent thermal conditions and adequate water supply. Meanwhile, the convective unstable stratification was formed over Shandong Province. Along the shear line at 850 hPa, the northeast wind with cold air and the lowlevel southwest jet with warm air converged. Because of the interaction between the two airflows in the low troposphere, the warm front had obvious characteristics of frontogenesis. The uplifting of warm air caused by the weak lowlevel cold air intrusion promoted the development of convection and energy release. The mesoscale convergence center or convergence line activated the mechanisms of rainstorm formation, and the distribution of heavy rainfall had the same direction with the ground convergence line. The strong rainfall intensity and the special terrain topography played an important role in this rainstorminduced disaster.