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Based on the NCEP 1°×1°reanalysis data and Tianjin Doppler data, the causes of the heavy rainfall happened in Tangshan and Tianjin between 14 and 15 July 2008 is analyzed, and the simulation with the PSU/NCAR MM5 model is made to verify the precision of the analysis. The results show that the heavy rainfall event occurred under the background of the interaction of mid and lowlatitude systems, caused by the upper trough on 700 hPa, low vortex on 850 hPa, and mesoscale convergence line in the boundarylayer. The vapor was delivered mainly by the low jet. The lowlevel convergence, upperlevel divergence, and lifting motion near the front were dynamical mechanisms that make the low vortex developed. The pulsatory strengthening of the southwest jet stream caused by the cold air from the midlevel was related to the amplifying of heavy rainfall. The major precipitation echoes of the heavy rainfall were eddy and band echoes.