A clamping mechanism includes a mounting member, three sealing assemblies, three clamping assemblies, and six adjusting valves. The mounting member defines a receiving hole axially and three mounting holes radially, the three mounting holes communicate with the receiving hole. Each sealing assembly includes a top seal and a bottom seal sealing opposite ends of corresponding mounting hole. Each clamping assembly includes a holding member and a piston, a first end of the holding member extends through the axial hole and is received in the receiving hole, the piston is fixed to the holding member and divides the mounting hole into a first chamber and a second chamber. The six adjusting valves respectively communicate with the first chambers and the second chambers of the three mounting holes.