Under accelerating global climate change, the increasing frequency of seasonal drought events in subtropical regions constitutes substantial challenges to the forest carbon sequestration potential and ecosystem stability. Consequently, elucidating hydraulic response mechanisms of subtropical secondary forest plants under drought stress holds crucial theoretical significance for predicting vegetation vulnerability to climatic extremes. Nevertheless, empirical evidence remains scarce regarding environmental determinants of xylem sap flow dynamics in secondary forest communities during extreme drought episodes. This investigation centered on three dominant tree species within a typical subtropical secondary forests ecosystem: Cinnamomum camphora, Pinus massoniana Lamb., and Liquidambar formosana Hance. Employing the thermal dissipation probe methodology, we implemented continuous stem sap flow monitoring throughout 2022, concurrently measuring meteorological parameters and soil moisture dynamics. A comparative examination was conducted to assess stem sap flow responses to meteorological variables and soil moisture conditions during the 2022 extreme drought event versus the hydrologically typical 2023 counterpart. The results reveal that: (1) The sap flow rates of the three tree species during the 2022 extreme drought period were significantly lower than those observed during the same period in 2023. Among the species, C. camphora exhibited the most pronounced reduction in sap flow rates during the transition from wet to drought conditions, followed by L. formosana, while P. massoniana showed the smallest decline. (2) Diurnal variation patterns revealed that during the 2023 drought period, the sap flow rates of all three species exhibited a clear diurnal cycle, with higher daytime sap flow rates compared to nighttime. In contrast, during the 2022 extreme drought period, C. camphora and P. massoniana displayed higher sap flow rates at night than during the day. (3) Correlation analysis between sap flow rates and environmental factors indicated that, irrespective of whether the year was wet or dry, during drought periods, the sap flow rates of all three species were positively correlated with solar radiation (Rn) and vapor pressure deficit (VPD), while negatively correlated with soil water content (SWC). However, the sensitivity of sap flow rates to environmental factors during the 2022 drought period was lower than that observed during the same period in 2023. (4) In relatively wet years, sap flow rates were primarily influenced by VPD and Rn, whereas in extreme drought years, soil moisture availability became the dominant limiting factor. This shift indicates a transition in the regulation of sap flow rates from being driven by climatic factors to being constrained by soil water availability under drought conditions.