The two extreme torrential rain， in the warm area before the cold front on 7 May 2018 and the South China monsoon trough torrential rain without cold air on 29 August 2018, were generated in the coastal linear mesoscale convective system (MCS) in Guangdong and Southern Fujian. After comparative analysis of the environmental background and the characteristics of the convective system， the main conclusions are presented as follows. In both cases, there were obvious 850 hPa and 925 hPa low-level jets. During the torrential rain on 7 May, the middle and lower troposphere had obvious unstable conditions. The significant baroclinic atmosphere, relatively high CAPE value， medium to strong 0-6 km deep vertical wind shear and the obvious dry layer in the middle troposphere were conducive to heavy rainfall and thunderstorm. On 29 August, the conditions were only weak instability, quasi-barotropic atmosphere, higher melting layer height, relatively low CAPE value, weaker 0-6 km deep vertical wind shear and high relative humidity across vertical layers, which were very conducive to heavy rainfall rather than thunderstorm gale. The mean winds in storm bearing layer of the two processes were both from the southwest, and the advection in the former was much stronger than that in the latter. The heavy rain in Xiamen and its surrounding Southern Fujian area was caused by backward propagation, which caused a number of strong convective rain masses to move over the same area and forming heavy rain. The backward propagation on 7 May was caused by the gust front of one convective rain mass meeting with the horizontal convective roll behind another convective rain mass, triggering a new convection. The new convection came from the land. In the case on 29 August, the warm and humid air flow in the lower layer met the gust front behind the mature convective rain mass, triggering a new convection. The new convection was located at sea and continued to move onto the land. Both the ice-phase process and the warm cloud process played a strong role in the convective system on 7 May, while the convective system warm cloud precipitation dominated on 29 August. During the two torrential rains， the meso-γ scale vortex formed near the gust front in the front of the cold pool of the convective system, and combined with the convergence and upward motion of the gust front, produced a positive vertical helicity, which was beneficial to the maintenance of the convective system. The low-level jet, through its transport of water vapor and heat, interaction with topography and cold pools of the convective system, played an important role in the maintenance of the coastal linear MCS.