The invention discloses a wellbore formwork support device and a method for mounting and detaching the same. The wellbore formwork support device comprises a core rod and a plurality of telescopic support units. The telescopic support units sleeve the core rod and are sequentially connected with one another; the telescopic support units structurally comprise inner hollow rods, the inner hollow rods sleeve the core rod, and four outer barrels are sequentially arranged along each inner hollow rod from bottom to top and are connected with one another by elastic mechanisms; support oblique rods are connected onto the first outer barrels, one end of each support oblique rod is movably connected to the middle of a support transverse rod, one end of each support transverse rod is fixed onto the corresponding second outer barrel, and the other end of each support transverse rod is fixed onto a formwork; the third outer barrels and the fourth outer barrels are arranged in the same manner; fixing fasteners are arranged on the first outer barrels and the second outer barrels. The wellbore formwork support device and the method have the advantages that narrow and deep wellbore inner form mounting and detaching operation can be implemented, accordingly, manpower and material resources can be saved, construction processes are simple, the wellbore formwork support device is convenient to mount and detach, and the construction time can be shortened.