Abstract: A series of thermo-responsive core cross-linked star polymers (CCSP), in which poly(methoxypolyethylene glycol-co-di(ethylene glycol) ethyl ether acrylate) (P(PEGA-co-DEGA)) served as the thermo-responsive arm, have been successfully prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization using the " arm-first” approach. Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (¹H-NMR) have been applied to characterize the structure and composition of the resultant polymers. The molecular weight of the P(PEGA-co-DEGA) arm and the corresponding CCSP were obtained by gel permeation chromatography (GPC) analysis, which showed that the efflux time of CCSP was significantly shortened compared with its corresponding arm, indicating that the CCSP was successfully prepared. The phase transition behavior of the resultant CCSP was evaluated by UV-Vis spectroscopy. Results showed that the low critical solution temperature (LCST) of CCSP displayed a downward trend with increasing the content of DEGA in the P(PEGA-co-DEGA) arm. The LCST of CCSP was also influenced by the pH value of the solution due to the ionization of the carboxylic acid groups at the end of the polymer chains, leading to an upward trend with the increase of pH value. The experimental results indicated that the LCST of CCSP could be controlled in the range of 49—82 ℃ by adjusting the molar ratios of PEGA to DEGA in the arm and the pH value of the solution. Furthermore, the prepared CCSP demonstrated a narrower temperature range of phase transition, thereby resulting in a faster phase separation process in comparison to the corresponding linear arm.