The concurrent occurrence of rainfall and heat in the subtropical monsoon climate promotes the formation of karst carbon sinks. Border poljes, recharged by allogenic water, are regions with strong carbonate rock dissolution in karst areas. Studying the hydrochemical characteristics of different water types in these regions helps deepen the understanding of karst carbon sequestration processes. This paper employs statistical methods, Piper diagrams, Gibbs diagrams, and major ion ratios to analyze the hydrochemical characteristics, origins, and carbon sink effects of rivers in a typical subtropical border polje. The results show that: (1) The main types of river water in the border polje are allogenic water, surface rivers recharged by allogenic water, karst underground rivers recharged by allogenic water, and karst groundwater. The total ion concentration shows an increasing trend. The average concentration of Ca²⁺ in karst groundwater is 91.06 mg/L, which is 2.3, 4.3, and 12.4 times that of underground rivers recharged by allogenic water, surface water recharged by allogenic water, and allogenic water, respectively. (2) After the allogenic water enters the karst area, the concentrations of Ca²⁺ and $\mathrm{HCO}_{3}^{-}$ gradually increase, and the calcite saturation index shifts positively. In the context of an accelerated water cycle due to heavy rainfall in the subtropical monsoon region, the reaction time of allogenic water with carbonate rocks is insufficient after entering the karst area to strengthen erosion. Whether it is direct infiltration of allogenic water, or karst underground rivers or surface water recharged by allogenic water, it has significant potential for increasing the carbon sink.