Targeting the severe torrential rain process in Henan Province on 20 July 2021, this paper focuses on revealing the features of the water vapor transport, the budget of water vapor flux and the transformation of water vapor as well as the associated impact on the variation of local precipitation based on FNL reanalysis data, gauge precipitation data and high-resolution simulation results. The results show that the water vapor over the ocean was continuously transported to the severe precipitation area by the water vapor channel built by the synergistic effect between the powerful and quasi-stationary Typhoon In-Fa over the East Sea and the stable atmospheric circulation pattern in the middle and high latitude zone. Then the local water vapor constrained within the extreme precipitation area was further transported from the low level to the middle and high level depending on the dynamical convergence mechanism in the low-level troposphere, which was dominated by the mesoscale synoptic system and the blocking effect of the windward slope. The vertical transport of the water vapor was indeed favorable for the reinforcement of the convection in the extreme precipitation area. Finally, under the synergistic effect of the convective ascending motion and the orographic lifting mechanism of the windward slope, more water vapor was vertically transported to the middle and higher troposphere by the reinforced vertical airflow, which stimulated the desublimation process between the water vapor and the ice-phase particles, resulting in the remarkable increase of the precipitation efficiency. The remarkable increase of the precipitation efficiency within the windward slope region was directly responsible for the occurrence of the severe torrential rain in the perspective of microphysical processes. This paper not only comprehensively reveals the main source and the consumption process of the local water vapor dominated by the muti-scale synoptic system in the extreme precipitation event, but also advances the understanding to the occurrence mechanism of the extreme precipitation.