Abstract: During the period 3-5 August 1996("96·8"), an extraordinary heavy rainfall event occurred in Henan, Hebei and Shanxi Provinces in China, resulting in severe flood catastrophe. Synoptic analyses indicate that the stable gross colfield and the interaction between a northward moving typhoon(down into low pressure) and its east lateral Pacific subtropical high were the large-scale and mesoscale circulation conditions of the occurred "96·8"extraodinary heavy rainfall; The mesoscale typhoon-low pressure and its specific dynamical and therm odynamical structures were directly related to this heavy rainfall event. Using the nonhy drostatic version of mesoscale numerical model MM5 to conduct investigation of numerical simulation for this case, the simulation of the full physics processes of nonhydrostatic version MM 5 was basically possessed of a capability to reproduce the genesis, development and evolution of the large-scale and meso-α scale weat her systems. The simulative results using a two-way interactive nesting procedure revealed that the typhoon-low pressure possessed an intensive coupled mechanism between the dynamical and thermodynamical fields, namely, the developing typhoon-low was possessed of a structure of the column of cyclonic vorticity with warm center and high humidity, within the lower levels of the vorticity column were associated with the moist convective instability and negative moist potential vorticity structure; the intensive ascending motion and the intense convergence (divergence) on the lower (upper) levels as well as the development of the convective cloud cluster were intercoupling; the intense south wind jet stream below 500 hPa accompanied by the typhoon-low was not only the interaccompany and intercoupling condition of the development and maintenance of the typhoon-low and convective cloud cluster, but also was the moisture source and heat energy transport belt of the "96·8" extraordinary heavy rainfall. The simulative results of precipitation show that the rain-band and rainf all dist ribution of precipitation with that of the observation were basically consistent in spite of some rainfall centers less or larger than those of observations for coarse or fine mesh, respectively.