Abstract: Biphasic absorbents have gained great attention in the field of flue gas CO2 capture for their potential to significantly reduce regeneration energy, yet current formulations struggle to balance absorption-regeneration performance with viscosity and lack long-term cycling validation. Here we employ stepwise screening and incremental optimization to develop a DETA/DEA/DMAC/H2O (DDDH) biphasic absorbent. The CO2-rich phase loading of DDDH solution attains 4.64 mol/kg, the viscosity is 16.02 mPa·s at 60 °C, and the cyclic capacity reaches 2.08 mol/kg, which was 38.7% higher than that of 30 wt% MEA. Potassium antimony tartrate was identified as the optimal oxidative-degradation inhibitor for DDDH solution, lowering the amine degradation ratio by 20.7% over four weeks. CO2 reaction pathways and the phase separation mechanism were elucidated by 13C NMR and quantum chemistry calculations. To assess operational stability of the biphasic absorbent, the DDDH + potassium antimony tartrate system was tested in a 4 Nm3/h absorption-desorption continuous flow system. By varying liquid-to-gas ratio and regeneration temperature, the impact of various operation condition on absorption-regeneration performance of biphasic solvent was revealed. Under optimized conditions, a 72 h continuous cycling operation achieved a CO2 removal efficiency >94.35%, cyclic capacity >2.04 mol/kg, regeneration energy <2.14 GJ/t CO2, and total amine loss ratio<1.61%. Therefore, the DDDH system exhibits good prospects for practical application for its excellent balance of low energy consumption and operational stability.