The main studying objects were the severe rainstorms which had happened since 1980's and aroused grave mountain torrents and mud-rock flows in the mountainous area in Liaodong peninsula. The formation reasons of severe rainstorms were inquired in suitable disposition and interaction,environmental conditions and physical mechanism for weather systems of large,middle and small scales,at high,middle and low latitudes and altitudes. It was shown that there is no obvious difference in thermal condition between the severe and normal rainstorms,and both need quasisaturated atmospheric layer resulted from the transportation and convergence of low altitude warm and moist air. The most remarkable feature is the dynamic condition of the violent vertical upward motion caused by the superposition of the violent convergence in the low level and the divergence in the upper air.