Abstract: The temporal and spatial high resolution data outputted from the Mesoscale Model(MM5V3)was used to diagnose convective momentum transport(CMT)on "99.6" Meiyu front rainstorm.The major conclusions as following:(1)The comparisons between results of simulated and observed shown,MM5V3 has the ability to replicate the large-scale and mesoscale circulations of the "99.6" Meiyu front rainstorm.(2)The diagnosis of CMT on the violently developing period of the mesoscale system show that the direction of momentum budget residual was consistent with the mean flow and acted to accelerate on the northward of southwest flow which favoring the genesis and development of the meso-Bsystems.It also shown that strong X always related to the meso-β systems.The distribution of kinetic energy transfer E suggested that the area of E>0 and EE was much greater than the absolute negative E which suggested the energy was mainly transferred from the large-scale motion to subgrid scale eddies,hence induced the genesis and development of subgrid scale systems.(3) X strong deceleration on the southwest flow occurred when the low vortex with shear line move to the sea.The analysis of v and v Y's cross sections indicates that the southerly was decelerated on low and middle levels of troposphere.(4) The vertical profile of Eindicates that the strong downscale transfer of energy (E>0) mainly occurred in the middle and low levels of troposphere during the intense development of mesoscale systems and upscale transfer (E<0) occurred when the low vortex move to the sea.