Abstract: Affected by the residual vortex of Typhoon Haikui (2311), an extreme rainstorm occurred along the southeast coast of Guangxi from September 10 to 11, 2023. The event featured intense rainfall, overlapping precipitation zones, and distinct nighttime rainfall characteristics, with many stations recording precipitation amounts that broke historical records. Based on multisource observations and ERA5 reanalysis data, the causes of the extreme rainstorm and the possible mechanism underlying the maintenance of the residual vortex were analyzed. The findings are as follows. Under the influence of a weakening continental high and a stable western Pacific subtropical high, the “Haikui” residual vortex, positioned between the two systems, was weakly guided by the steering flow and remained stationary over Guangxi. Working in tandem with a strong southwest monsoon flow, this condition led to the extreme rainstorm. The area near the rainstorm center exhibited a strong potential for heavy precipitation, and the precipitable water content was abnormally high before the peak rainfall. The boundary layer jet on the southeast side of the residual vortex served as the key influencing system for the extreme rainstorm, playing a cross-scale pivotal role in linking the residual vortex with local severe precipitation. Its nocturnal development was associated with an increase in local kinetic energy driven by the work done by the meridional pressure gradient force in the southeast quadrant of the residual vortex. The negative meridional geopotential height gradient on the southeast side, coupled with southerly winds, enhanced local kinetic energy and promoted the nighttime intensification of the boundary layer jet. The evolution of the boundary layer jet facilitated the formation of strong vertical helicity through low-level convergence uplift combined with the positive vorticity circulation of the residual vortex. Meanwhile, the frontogenesis forcing of convergence and deformation at low levels provided dynamic conditions favorable for heavy precipitation. Additionally, the rainstorm center maintained high temperature and humidity at lower levels and a deeper wet layer. The presence of a deep warm-advection heating layer and latent heating in the mid-troposphere created favorable thermal conditions for sustaining the residual vortex circulation. At night, perturbation downslope winds along the eastern slope of the Yunnan–Guizhou Plateau and perturbation onshore winds from the Beibu Gulf converged toward the center of the residual vortex. This interaction enhanced wind field convergence and provided dynamic conditions for the development and maintenance of cyclonic vorticity within the residual vortex.