To gain more knowledge on the transformation process of dissolved inorganic carbon in a karst reservoir, in situ monitoring, sampling and lab experiments of water columns were carried out at various sampling sites along the flow line in Wulixia Reservoir, Guangxi, China during early July, 2013. Results showed that: (1) The hydrochemical characteristics of study areas were controlled by the carbonate equilibrium system and the hydrochemical type of all water samples was HCO3-Ca + Mg. (2) The DIC concentration decreased gradually (DIC(Average) : from 1.03 to 0.78 mmol x L(-1)) and the delta13C(DIC) increased gradually (delta13C(DIC(Average) : from -10.21per thousand to -6.62 per thousand) from the reservoir end area to dam area. Meanwhile, with the depth increase in water column, the DIC concentration increased gradually (DIC(Average) : from 0.86 to 1.05 mmol x L(-1)) and the delta13C(DIC) decreased gradually (delta13C(DIC(Average) : from -7.88 per thousand to -13.39 per thousand) from the surface to the bottom of the reservoir. Possible reasons for these research results were found as follows: (1) Dissolution-precipitation process of carbonate substance could be inhibited by other processes such as biogeochemical processes, which played little role in delta13C(DIC) variations. (2) Thermal stratification existed in the study areas which could influence the distribution of DIC and delta13C(DIC) by affecting the distribution of plankton and its orientation and strength of metabolism process, and the extent of organic matter decomposition, and so on.