Abstract: Heavy precipitation in Guangxi exhibits pronounced spatial variability. However, detailed comparative studies on the differences in moisture mechanisms among precipitation modes within the same event are still lacking.To address this gap, multi-source observations and reanalysis data are used for a diagnostic analysis of a heavy precipitation event in Guangxi during 18-19 May 2024, based on moisture budget and apparent moisture sink diagnostics, backward trajectory tracking, and clustering. This study compares the moisture characteristics of three precipitation modes: extreme heavy precipitation in the coastal warm sector, low-vortex-related widespread heavy precipitation in central Guangxi, and stable precipitation associated with weak cold-air intrusion in northern Guangxi. From a moisture perspective, this comparison reveals possible mechanisms for the spatial differences in precipitation location and intensity. The results are as follows. (1) The precipitable water vapor (PWV) shows a stepwise decrease from the coastal region to central Guangxi and then to northern Guangxi, consistent with precipitation intensity. Compared with other regions, PWV in the coastal region shows a more pronounced increase before heavy precipitation. (2) The coastal region has a favorable vertical configuration, with strong low-level moisture transport, strong near-surface convergence, and deep upward motion. accompanied by positive feedback from condensational latent heat release, which is most favorable for converting moisture into precipitation. followed by central Guangxi. In northern Guangxi, low-level moisture supply and upward motion are both weak. The latent heating layer is higher and overlaps with mid-level divergence, which interrupts the positive feedback. (3) In the low-level moisture budget, net inflow is largest in central Guangxi, followed by the coastal region, and weakest in northern Guangxi. The net inflow layer is deep in the first two regions. In northern Guangxi, it is the shallowest due to cold-air lifting. (4) In terms of moisture source contributions, the coastal region is dominated by a low-thermal-energy, moist channel from the offshore waters of eastern Guangdong. This channel, together with relatively high-thermal-energy, moist air masses over the northeastern South China Sea, creates a certain low-level thermal contrast. Central Guangxi is affected by the superposition of two high-energy warm, moist channels and has sufficient energy. Northern Guangxi has multiple scattered channels and weak thermal conditions. This study provides a region-specific moisture diagnostic approach for refined forecasting of different precipitation modes in Guangxi.