An analysis on the heavy rain in central and southern Shaanxi fr om 8 to 9 A ugust 2007 is performed with the routineobservational data provided by MICAPS, surface data of intensive observation, FY2C satellite TBB. The results indicat e that the mesoscale shear line on 500hPa and the low vortex on 700hPa are its ma inly influencing systems, and it is directly caused by MCC. The spatial and temp oral variations of moist potential vorticity can give good indication for the de v elopment of heavy rain. Superimposition of the positive and negative moist poten tial vorticity is advantageous to the development of the rainstorm. The heavy ra in is located near the zero line of positive potential vorticity and in tensive belt of negative MPV1 on 700hPa. The strong convergence in the east side of low vortex on 700hPa and strong divergence in the right side of high altitud e jet stream at 200hPa supply strong and continuous ascending movement.