Diagnostic analyses are carried out for the rainstorm from 2000 BST 5th to 0800 BST 12th July 2005 and WRF model is applied to simulate the precipitation process from 0800 BST 6th to 2000 BST 7th July 2005.Results show that: (1)The rainstorm process can be divided into two periods,nonlinear subcritical symmetric instability might be the main mechanism. (2)The strengthening of zonal disturbance wind component u and meridional v are respectively in advance of precipitation’s enhancement,and the peaks of u and v disturbances as well as peaks of ageostrophic intensity and cold or warm air’s peak indicate the precipitation peak which emerges after 12 or 24h. (3)The space-time allocation among peaks of upper jet,lower jet,upper disturbance wind,lower disturbance wind and precipitation is revealed. (4)It is depicted that at earlier stage of rainfall,space-time allocations relate among upper level vorticity,divergence and lower level vorticity,divergence.The new physical mechanism that nonlinear subcritical symmetric instability stimulates rainstorm is shown by the above 4 points and the situation of vertical upward current.