Based on NCEP/NCAR FNL reanalysis data(1°×1°), CMORPH-AWS merged hourly gridded precipitation product, and FY-2G TBB (black-body temperature), the mechanism of the torrential rain caused by Typhoon MANGKHUT (2018) is analyzed. The results show that the maintenance of the western North Pacific subtropical high and the South Asia high is beneficial to the continuous influence of typhoon residual vortex on South China. The low-level jet stream from the Bay of Bengal converges with the easterly wind on the south side of the subtropical high, setting up a water vapor channel accessing South China. The apparent heat source and apparent vapor sink are mainly generated with latent heat release of condensation by vertical motion under the influence of Typhoon MANGKHUT.The diagnosis of moist potential vorticity (MPV) indicates that vertical shear of strong horizontal wind leads to the enhancement of baroclinicity at lower level and significant convective instability.