Abstract: In order to explore the spatial characteristics of reservoir hydrochemistry and dissolved inorganic carbon in karst geological environment. Pingzhai Reservoir is selected as the study area which is the water source of Qianzhong Water Conservancy Project in Guizhou Province located in typical karst area. Stratified sampling of Pingzhai Reservoir is conducted during the summer thermal stratification period (July 2019), analyzing the main physical and chemical data and dissolved inorganic carbon isotope of water bodies. The results show that: 1) Pingzhai Reservoir has obvious thermal stratification in summer: surface water layer (1~-10 m), thermocline (-10~-30℃ m), constant temperature layer (-30~-60 m). The water temperature decreases with the increase of depth, and finally stabilizes (T (average): 25.62~12.43℃). The pH, dissolved oxygen (DO), electrical conductivity (EC), chlorophyll a (Chl-a) and other physical and chemical properties of water also show obvious stratification in vertical direction. 2) The hydrochemistry type of Pingzhai Reservoir is HCO3+SO4-Ca, and its hydrochemistry is mainly controlled by the carbonate balance system; 3)Change of dissolved inorganic carbon. (DIC) and its isotopic composition (δ13CDIC) in water: DIC concentration gradually increased with the increase of water depth (DIC (average): 1.8~3.03℃mmol/L), δ13CDIC shows a slight trend (δ13CDIC (average): -6.23‰ ~-11.45‰). According to the analysis, solar radiation, the distribution of aquatic organisms, photosynthesis, respiration intensity and the degree of decomposition of organic matter are different at different depths, resulting in significant stratification of the physicochemical properties of water and DIC, which further affects the fractionation of δ13CDIC. The dissolution and precipitation of carbonate rocks have little effect on the content of DIC and its stable isotope fractionation.