In the context of global climate change, drought events significantly impact plants, and studying the response of plant sap flow to environmental factors is of great significance for plant protection and sustainable development. This article uses the thermal pulse method to obtain trunk sap flow data, combined with real-time monitoring of environmental factor data, to compare the differences in sap flow characteristics between the subsequently deceased and surviving Zanthoxylum planispinum var. dingtanensis during a natural drought event. Additionally, the xylem vulnerability curve of Z. planispinum was fitted through laboratory measurements to estimate its hydraulic safety margin ($HSM$). The results showed that: (1) Compared with the surviving individuals, the subsequently deceased Z. planispinum exhibited a higher and more fluctuating sap flow rate, maintaining a consistently higher transpiration rate even after the onset of drought. (2) As soil moisture decreased, the sap flow of surviving Z. planispinum was promptly constrained by soil water availability and remained at a low, conservative level. In contrast, the sap flow of the deceased individuals continued to be driven by meteorological factors, failing to downregulate transpiration. (3) The xylem vulnerability curve revealed a $P_{50}$ of −2.2 MPa. During the severe drought, the minimum field branch water potential (${\psi }_{min}$) dropped to −3.0 MPa, resulting in a negative $HSM$ ($HSM$ = −0.8 MPa). This suggests that the xylem tension severely breached the embolism threshold, likely triggering catastrophic hydraulic failure, which emerged as the primary driver of the observed mortality in Z. planispinum. Therefore, for plants with low $HSM$s, when they are subjected to drought stress accompanied by highly fluctuating sap flow—a condition indicating that water supply cannot match the transpirational demand—timely manual intervention (e.g., supplemental irrigation) may be considered to mitigate hydraulic risks and ensure survival.