Ewiniar (1804), a tropical storm with weak tropical cyclone (TC) intensity but strong and long lasting precipitation, is taken as the research object in this study. By using the conventional and unconventional observations, and NCEP analysis data, the causes of persistent heavy rainfall of the “weak” TC are analyzed. Ewiniar moved very slowly due to the very weak steering flow caused by the large scale circulation background. As a result, its TC circulation maintained for a long time over the offshore waters in South China. Water vapor and momentum were transported to the TC circulation continuously by both the abnormal strong southwest jet induced by the onset of the South China monsoon and the easterly jet caused by Maliksi (1805). Diagnosis of net water vapor budget shows that in the early stage, water vapor inflow mainly came from Ewiniar’s southern boundary, and then water inflow in the eastern boundary increased rapidly, which played an important role in maintaining the continuous water input in the whole TC circulation. As a result, the water flux convergence and high humidity and high energy environment along the coastal areas of Guangdong maintained. Strong southeast onshore wind existed for a long spell along the coastal areas of Guangdong. Impacted by the costal line and topography, sustained convergence can be found in lower-level along coastal areas, which could provide favorable dynamic conditions for the continuous generation of heavy rainfall and triggering of mesoscale convective system. And the enhancement of the southeast wind speed in lower troposphere was in good agreement with the sharp increase in precipitation. There were active mesoscale convective systems along the coastal areas, accompanying the occurrence and development of five-stage mesoscale convective rain belts. The continuous generation of new convections in the coastal areas, the development of mesoscale spiral rain band, the superposition of TC inner core precipitation and convections in the outer rain belt, as well as the migration of convection over sea from the two warm wet conveyor belts all contributed to the persistence of the mesoscale convective system in the coastal area of Guangdong Province, resulting in the generation of persistent heavy rainfall there.