The phase splitting solvent as a promising alternative to MEA for CO2 capture has received an increasing attention due to its low energy consumption. In this paper, the mixture of diethylenetriamine (DETA) and sulfolane was employed to absorb CO2 from the simulated flue gas. Its phase splitting behavior was investigated. The species in each separated phase were identified using nuclear magnetic resonance (NMR), revealing that the upper layer was a CO2-rich phase and mainly consisted of DETA and DETA-carbamate. Whereas, the lower layer was CO2-lean phase with the sulfolane presented individually. The CO2 absorption kinetics was also measured in a wetted-wall column as a function of temperatures, CO2 loadings, and gas flow rate. Their impacts on the enhancement factor and mass transfer resistance were calculated by the Vapor-Liquid Equilibria (VLE) model, implying that the main CO2 mass transfer resistance gradually transferred from the gas side to the liquid side with the CO2 loading increasing from 0 to 2.21 mol L-1. During the CO2 absorption process, sulfolane acted as an inert solvent for substituting the role of water and separated from the absorption product. Consequently, the sensible heat and vaporization heat were substantially decreased by 59% and 12%, respectively. Hence the total heat duty was lowered by 19% which might noticeably advance the CO2 capture technology.