Based on Shuman Shapiro mesoscale filter,mesoscale analysis under the background of large-scale circulation is undertaken of the heavy torrential rain in Shandong Peninsula during August 11 to 12,1999.Results show that the rain can be attributed to the northward movement and development of a tropical mesoscale cyclone.The invasion of surface cold air to the westward mesoscale depression leads to the development of convective activities and to the development of the tropical mesoscale cyclone.The transportation of lots of water vapor in large-scale circulation provides the energy to the development and maintenance of the strong convection.The surface mesoscale depression center and the wave spot on the shear line is indicative to the rainfall area.