A novel hyperbranched lower critical solution temperature (LCST) polymer with sharp temperature and CO -responsive behaviors is presented in this study. The target polymer of hyperbranched poly(oligo(ethylene glycol) (HBPOEG) is constructed using POEG as the backbone and tertiary amines as branch points. Phase transition of HBPOEG in aqueous solution is investigated by heating and cooling the system; the results indicate that HBPOEG in aqueous solution has a concentration-dependent phase transition behavior with excellent repeatability. Moreover, LCST of HBPOEG can be tuned by bubbling CO into the solution, as the tertiary amines can be protonated and the solubility of the polymer would increase by bubbling CO into the system, leading to an increase of LCST of the polymer. Further bubbling N to remove CO can reversibly turn back the LCST to its original value. This backbone-based hyperbranched LCST polymer with both CO and temperature responsiveness can be applied in application areas like drug delivery, gene transfection, functional coatings, etc.