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The evolution of three mesoscale convective systems (MCS), i. e., MCS-A, MCS-B, MCS-C, experienced multiple division and reorganization, resulting in a heavy rainfall in the warm sector ahead of fronts in Southern China on 27 March 2020, which sustained for more than 15 hours. The synoptic background, organization modes and initiation conditions of MCS are analyzed by using surface automatic meteorological stations data, Doppler weather radar data, ERA-Interim reanalysis data and mesoscale CMAGD model data. The results are shown as follows. (1) Favorable configuration and stable circulation systems, including a 500 hPa upper-level trough, a 200 hPa westerly jet and a frontal trough are important reasons for long-term maintenance of the warm-sector heavy rainfall. The low-level jet provides conditional instability and convective available potential energy for the generation and development of convection. (2) The convection in key region 1 from Hezhou to Huaiji was triggered on the ground with the decrease of the level of free convection and the reduction of CIN energy. The interaction between wind speed convergence at the height of 1500 m and a cool pool in key region 2 80 km away from key region 1 resulted in the convection. (3) MCS-A and MCS-B are modulated into forward and backward propagation in key region 1, while MCS-C is modulated into backward propagation in key region 2. The convective cells of all three MCSs form echo "train band effects". In addition, both MCS-B and MCS-C form several short and parallel convective rainbands in a nearly N-S direction triggering eastward "train band effects". MCS-A and MCS-B split again into short and parallel convective rainbands in a nearly NE-SW direction triggering southeastward "train band effects". (4) The warm and humid southwesterly flow on the left side of the meso - β - scale secondary circulation is lifted, releasing latent heat and triggering convection. The interaction of the cold outflow resulting from the drag effect of heavy precipitation formed a downdraft below 850 hPa and a warm inflow on the west side near the surface was conductive to new convection, which was the development mechanism of the backward propagation. (5) Forward propagation was caused by the convergence between the cold pool outflow, which was formed on the ground after the genesis of upstream MCSs, and the warm moist flow. [ABSTRACT FROM AUTHOR]