During July 1-2, 2024, a regional torrential rainfall event occurred in Guangxi, characterized by extensive coverage, large cumulative rainfall, and obvious differences in the characteristics of the previous and subsequent stages. This study utilizes multi-source observational data, ERA5 reanalysis data, and numerical model forecast data to analyze the synoptic mechanisms. The results show that (1) The event consisted of frontal heavy rainfall over northern Guangxi and warm area heavy rainfall over southwestern Guangxi. The first stage is frontal heavy rainfall, which occurs in the northern part of Guangxi and is jointly affected by the upper-level trough, low-level shear line, and southwest low-level jet. The second stage is the warm area rainfall, which occurs in the south of Guangxi where the low-level jet continues to affect. (2) In the first stage, the east-west squall enters Guangxi from Guizhou and slowly moves southward, the convective system has strong organization and large east-west span. The stage has an obvious frontogenetic forcing, and boundary layer frontogenesis plays a dominant role in the movement of the rain belt; In the second stage of the process, the lower layer is controlled by southerly winds, forming a dual jet stream feature in the boundary layer and lower troposphere. At the same time, the low-level disturbance caused by the enhanced pulsation of the southerly jet stream in the boundary layer is strengthened, providing favorable conditions for convective triggering. (3) In both stages, conditional instability and high convective available potential energy (CAPE) in the middle and lower troposphere before convective initiation supported both the development and maintenance of convection. (4) Terrain forcing plays an important role in this process. In the first stage, the main upward motion in northern Guangxi is induced by flow over the windward slope due to orographic uplift. In the second stage, upward motion in southern Guangxi is primarily generated by boundary layer frictional convergence.