Abstract: Using Tσ as frontogenetical parameter, the frontogenetical equations which can be used in meiyu front are derived. Because horizontal temperature gradient is very weak, and the moist gradient is very strong in meiyu front, classical dynamical frontogenetical equations can not be used for diagnosis. Some cases of strong precipitation from Meiyu front over Jiang-Huai valley in 1991 are calculated. The results indicate that Meiyu front dynamical frontogenetic equations can be applied in these cases. There exist cross front secondary circulations in Meiyu front, and the circulations is nearly vertical. The cross front circulation from dry and cold regime can establish moist static stability. The frontogenetical (frontolytical) is caused mostly from ageostrophic metamorphonesis term, and it has a positive feedback relation with ascending motion of frontogenesis. The horizontal frontogenesis of Tσ in Meiyu front has well indicating action for future six hours precipitation.