An extreme rainfall event [maximum of 1056.7 mm (24 h)-1] induced by an ultra-long-duration, linearly-shaped mesoscale convective system (β-MCS) occurred over Gaotan town (GT) of the Guangdong Province on 30–31 August 2018. This event broke the highest record for the Guangdong Province, caused a severe flash flood, and created social concern. Using multiple observations and NCEP/NCER_FNL data, we performed an evaluation of the precipitation, convection, and environmental conditions as well as initiation and maintenance for the β-MCS. The data showed that the tropical cloud clusters moved northward and induced large-scale heavy rainfall, with a background of a monsoon depression in a mesoscale favorable environment. A linearly-shaped β-MCS, characterized with a back-building, ultra-long-duration, quasi-stationary, low echo-top-height, and low echo-convective-centroid was responsible for the record-breaking rainfall over GT. Analysis using a rotation rate equation of sea and land breezes indicated the convection initiation and organization are closely related to the near-surface flow affected by multiscale systems. Southerly flow was sustained for a long period that was determined by reverse forces between the monsoon depression, local terrain, and barometric gradient, while all three came to a balance. Strengthening of the southerly flow on the HJ river valley side over the terrain slope helped warm-ridge development of the temperature field, blocking cold pooling. The outflow boundary moved southeastward on the mountain side over the terrain slope, leading to a sharp temperature gradient in that region. Quantitative diagnosis using a mesoscale atmospheric dynamics equation demonstrated the dynamic mechanism sustaining the convection maintenance and β-MCS organization to local vertical wind shear, causing a sharp temperature gradient.