Karst dynamic system is a extremely open, sensitive and complex system. An important method on the research of dissolution rate and carbon cycle in karst dynamic system is hydrological and geochemical analysis under the influence of rainfall in a karst area. Strontium and its isotope (n(87Sr)/n(86Sr)) are important tools to trace dissolution rate, intensity and carbon sink in karst process. Achievements and studies were mainly on hydrogeological processes, groundwater solutes origin and anthropogenic activities through strontium research. However, very few studies have been conducted to distinguished the differences of dissolution rate and carbon sink flux triggered respectively by "old water" storage and fresh rainwater recharge in a karst dynamic system.Methods:Employing the highresolution monitoring and sampling methods, a typical karst dynamic system in subtropical area, south China was chosen to research diel variations of groundwater Sr and n(87Sr)/n(86Sr) in a karst spring and to calculate the dissolution rate in a storm rainfall. Insitu measurement of hydrochemical variables including temperature, pH and specific conductivity (Spc, 25℃) were performed at S31 spring outlet using multiparameters meters. HCO-3 was titrated immediately in the field using a portable testing kit by Merck KGaA Co. (Germany). For continuous streamflow data, weir water stage was continuously monitored at 15min time interval at S31 spring using a CTDP data sonde. Discharge was then calculated using the rectangular weirdischarge formula. Rainwater sample and filtered spring water samples were collected in prewashed high density polyethylene (HDPE). Major anions were measured by an automated ion chromatography. Major cations were analyzed by ICPOES (IRIS Intrepid II XSP, Thermo Fisher Scientific, USA). In addition, the water samples (500 mL) and a bedrock sample (limestone) were collected for the determination of n(87Sr)/n(86Sr). Isotope analyses were determined using a Finnigan MAT 261 multiple collector mass spectrometer. Partial pressure of CO2 (PCO2) and saturation index of calcite (SIc) were calculated by the program WATSPEC using hydrochemical data sets.Results:The results showed that both CO2 effect and dilution effect were the controlling mechanism in hydrochemical variation of spring water in the initial rainfall. Dilution effect became the main controlling mechanism with the spring discharge rise and rainfall continuance. The variations of Sr concentration in spring water were obviously impacted by dilution effect, while n(87Sr)/n(86Sr) variations were not almost impacted, which indicated that continuously karst processes in karst aquifer in rainfall event may maintained the almost stable n(87Sr)/n(86Sr) value of spring water. Conclusions:Results from the mass balance equation indicated that n(87Sr)/n(86Sr) shares originated from the limestone dissolution which triggered by fresh rainwater recharge was 24.3% of total n(87Sr)/n(86Sr) flux in spring water, and the net dissolution rate of CaCO3 (limestone) and carbon sink flux was 0.136 mg/(cm2·d) and 1.01 t CO2, respectively. These results preliminary distinguished the differences of dissolution rate and carbon sink flux triggered respectively by "old water" storage and fresh rainwater recharge in a karst dynamic system. These results also contribute to the exact calculation of dissolution rate and carbon sink flux that triggered by a rain event.