Based on the NECP/NCAR reanalysis data, highlevel and surface observation data and radar data, this paper analyzes the three stages of the heavy rainfall that occurred in Zhejing Province during 18-20 August 2014. The water vapor transmission, vertical movement and convective instability of the rainstorm are investigated from horizontal and vertical directions, so the lesson of a failure forecast could be summarized. The results are as follows. (1) The first stage of the heavy rainfall in northern Zhejiang was mainly caused by the cold front, but the existence of the tropical depression played a positive role in strengthening warm wet flow coming from southwest, making the north cold air meet with the warm wet flow, which induced the frontogenetic function in the deformation field. So, the neglected existence and influence of the tropical depression are the main inaccurate reason of the less precipitation forecast. In addition, the duration of the rainstorm was also underestimated. (2) The second stage of the heavy rainfall happened in the central and southern Zhejiang. The southerly and southwesterly flow around the eastern or southern side of the low pressure drew abundant water vapor. There was a slowmoving cold front that resulted in the maintenance of the mesoscale convergence line in the lower atmosphere for a long time when the tropical depression landed at Fujian Province. There existed the coupling of the two systems, generating strong upward motion, and the instability energy in the lower level released. Finally, the severe downpour occurred. So, the underestimated influences of the tropical depression (moisture, dynamic and instability energy conditions) are the main inaccurate reason for the less precipitation forecast. (3) The big error of numerical forecasting is a crucial reason for overestimating the precipitation in the third stage. As a result, we find that the duration of the rainfall, heat condition and correction of numerical forecasting played important roles in predicting the precipitation of such rainstorm. Besides, there was strong upward motion caused by the coupling of tropical weather system and midlatitude weather system, and meanwhile, severe precipitation area maintained at the same place when the two systems confronted each other. What is more, plenty of water vapor and energy brought by jet caused the heavy rainfall frequently. Therefore, the actual rainfall was bigger than the forecasted rainfall.