Torrential rainfall often occurs at the coast of South China, with background of frontal rain band over northern South China or the Yangtze River basin, but it is difficult to forecast. Based on multiple observation data and ERA-Interim 0.125°×0.125° interval 6 h reanalysis data, we researched coastal torrential rainfall event from 15 June to 16 June 2017. We contrasted the precipitation characteristics and ambient atmospheric conditions between frontal rain band and coastal rainfall, and analyzed convection initiation and maintenance mechanisms for the latter. Finally, dynamic effect of the convection maintenance was proposed. The results are as follows. (1) There was considerable discrepancy between frontal rainfall and coastal rainfall. The former is presented as stratification cloud rainfall and weak convective rainfall caused by synoptic scale system (cold front), and its intensity is heavier to the west than that to the east. The latter characterized with convective, local, concentrated, and long-lived, which occurred at midnight. (2) Horizontal water vapor advection and vertical water vapor transport within boundary layer accounted for discrepancy between frontal rainfall and coastal rainfall. Owing to convective instability in middle and low troposphere associated with warm and moisture transport, rain rate over coastal South China is notably higher than those under the east of frontal rain band. (3) Lateral friction and direct block from the Lianhua Mountains and the Emei Hills caused vertical vorticity develop in river valley Luohe, ultimately triggering updrafts and convection initiation. Horizontal vorticity developed with strengthening of vertical wind shear, while southwesterly jet in boundary layer enhanced at midnight, and horizontal vorticity then partly converted into vertical vorticity, overlying the vertical vorticity originated from lateral friction and direct block. The process above is responsible for the long duration of coastal rainfall as well as dynamic effect of convection maintenance. (4) Rainfall sustaining long duration is also associated with the coutinuous releasing of convection latent heat. Upward vertical velocity caused by thermal buoyancy related to convection latent heat accounted for 39%-75% of total updraft vertical velocity.