Abstract: A numerical simulation of a torrential rain event occurring in the Jiang Huai valley of China from June 22 to 24, 1999 by using the PSU/NCAR MM5 mesoscale non-hydrostatic model is analyzed, and the high resolution model output data are utilized to diagnose the double front structure, and the relevant potential temperature，equivalent potential temperature and specific humidity distributions in the vicinity of the meiyu front system (MYFS) in the Jiang-Huai valley. The results show that both the potential temperature gradient and specific humidity gradient have important impact on the two strong equivalent potential temperature gradient zones associated with the double front structures of the MYFS, but the latter is more important to the structure. Then the tendency equation of specific humidity gradient is theoretically derived, and it shows that the variation of specific humidity gradient is related to the advection, divergence/convergence, horizontal and vertical vortex tube (secondary circulation) effects and the gradient of water vapor source/sink. As an example, the budget of the meridional component of the trend equation is selected and diagnosed by using the above mentioned mesoscale simulation data of the torrential rain in the JiangHuai valley, and the diagnostic results show that the temporal variation of the averaged specific humidity gradient over the duration of the simulation is mainlyrelated with convergence/divergence effect, secondary circulation effect associated with horizontal vortex tube, and water vapor source/sink effect. Since the water vapor source/sink is often resulted from the phase change processes associated with the water vapor in air and thus directly related with the development and evolution of cloud and precipitation microphysical processes. Therefore, the variation of specific humidity gradient is close related to the distribution, development and evolution of cloud and precipitation systems. The double front structure of the MYFS provides advantageous environmental conditions for the development and movement of the mesoscale torrential rain systems nearby, thus organizing and controlling the evolution of the systems, and in turn, the development and evolution of the relevant cloud and precipitation systems exert important impact on the MYFS through changing the thermal and moisture distributions.