By mesoanalysis of a precipitation that occurred in the middle reaches of the Changjiang River, 8th Jul. in 2003, the formation and development of the event, its relation with the meso-microscale system are studied, it is emphasized that the low-level jet would cause favorable thermodynamic, moisture and dynamic conditions for the heavy rainfall in the middle reaches of the Changjiang River and that the mesoscale vortex formed in the left of the low-level jet would stimulate strong convective clouds and cause the heavy rain.