Diagnostic analysis on the torrential rain process on 9 July 2012 in southern region of Shandong Province was made by using conventional meteorological data，the intensive observations of automatic weather station， FY－2E satellite images，NCEP reanalysis data and Doppler weather radar．The results show that the heavy rain occurred under the circulation form of two high confrontation ．The location and intensity of the upper and lower－level jet streams influenced rainstorm area and rainfall intensity． The occurrence and development of ground meso － scale cyclone was the direct cause of the torrential rain．The strengthening of the high pressure system over the west of Lake Baikal guided the cold and dry air from its north into southern area of Shandong，which cased ground convergence to strengthen and triggered the initial meso － scale cyclone generating． Cold and warm air over the storm area merged and triggered the release of unstable energy，which directly resulted in the torrential rain． The VWP product of Doppler radar could be a good catch for cold and warm air activities．