In August 27-28, 2004, a local rainstorm occurred in Dalian, along with strong wind, caused severe damages. The chara cteristics and forming causes of this rainstorm is studied through the analysis of daily observation data and detecting data from satellites, radars and automatic stations. It is found that this rainstorm was induced directly by three mesoscale and smallscale systems under the condition of steady largescale circulation. The basic causes of this rainstorm were the steady western Pacific subtropical hight and the stable and strengthening high pressure ridge in the east; meanwhile, caused by highlevel front positive vorticity advection, the pressure of the middle and lower troposphere dropped resulted in drastic climbingup of the air current. The riseing of the warm and humid current in the lower atmosphare contributed to the bursting of convective unsteady energy, and caused the local strong convection and the rainstorm.