[Objective] The impacts of soil erosion on reservoir carbon sinks in karst watersheds were studied, in order to provide scientific basis for regional soil erosion control and scientific assessment of carbon cycle in river ecosystems in karst areas.[Methods] Based on hydrological data of Nayong River from 2014 to 2018 and water chemistry monitoring data of Pingzhai Reservoir at the downstream from 2017 to 2018, regression analysis, correlation analysis, and water chemistry analysis were used to study the effects of soil erosion on reservoir carbon sinks.[Results] ① During period from 2014 to 2018, the flow of the Nayong River ranged from 1.151 to 1.663 billion m³/a, and sediment flux ranged from 25.36 to 1.224 7 million t/a. Sediment flux and water flow were not correlated between years but showed a highly significant positive correlation on the monthly time scale. The short-time intense rainfall was one of the main reasons for the occurrence of soil erosion in the watershed. The erosion was intense in spring and summer and weak in autumn and winter. ② Ca²⁺ and HCO₃^- were the main anions in Pingzhai Reservoir, which came from carbonate rock weathering. SO₄^2- was the second most important anion, participating in 30.30%~59.70% of carbonate rock weathering in the reservoir. Reservoir water chemistry type was HCO₃^--Ca type in autumn and winter, and mainly HCO₃^--SO₄^2--Ca type in spring and summer. ③ The partial pressure of carbon dioxide in the reservoir of Pingzhai Reservoir showed spatial and temporal heterogeneity as:winter > autumn > spring > summer, which were showed as carbon sources during autumn and winter, and as carbon sinks while spring and summer.[Conclusion] Soil erosion and reservoir carbon sink effect in Pingzhai Reservoir basin have the same periodic seasonal changes, soil erosion affects reservoir water chemistry and weakens reservoir carbon sink effect.