The invention relates to a pressurization expansion mechanism used for an expansion hanger. The pressurization expansion mechanism comprises a central pipe, an expansion tool, a fixed spline shaft, a spline, an inverted connector and a left hand fishing spear and a pressurization central pipe, wherein the upper portion and the lower portion of the fixed spline shaft are connected with the pressurization central pipe and the central pipe respectively, a liquid inlet hole is formed in the middle of the fixed spline shaft, the expansion tool is located below the fixed spline shaft and is matched with the central pipe in a sliding sealing mode, the spline is arranged on the upper portion of the fixed spline shaft, the fixed spline shaft is axially connected with the left hand fishing spear in an inserted mode and matched with the left hand fishing spear through the fixed spline shaft, the upper portion of the left hand fishing spear is axially matched with the pressurization central pipe in a sliding sealing mode, and left hand threads connected and matched with an expansion hanging cylinder are arranged on the lower portion of the left hand fishing spear. The pressurization expansion mechanism has the advantages that the expansion hanger is hung in a sit mode through the hydraulic pressure and machine combined expansion method, and the construction reliability is improved.