A heavy rain occurred in the eastern edge of Tibetan Plateau during August 25 － 26 of 2007 resulted in serious disaster． The
weather system and vapour transportation was analyzed by using NCEP( 1° × 1°) data，and then the PV theory was used to study the
mechanism of the heavy rain． The results show that the strong vapour transportation in the edge of the subtropical high and vapour convergence
over the rainstorm area was the important reason for the rainstorm occurrence． The strong instability before the rainstorm provided
benefit thermal condition． The northwest cold low trough had not obvious sign on the pressure field in the course of the rainstorm，
which only formed a weak wind shear near the rainstorm area，and it had small effect on this heavy rain． But the 325 K isentropic surface
had obvious indicating meaning for the rainstorm occurrence，when the cold air with higher PV value slided down to the low level
along the steep isentropic surface，the vertical vorticity increased rapidly because of the conservation of PV，as a result，the heavy rain occurred．