The invention discloses an underground construction fabricated confined concrete lagging jack and a mechanized construction method. In order to be matched with lagging jack intelligent mounting equipment to conduct tunnel construction, the fabricated confined concrete lagging jack is researched and developed, the relevant technologies of folded hoisting, mechanized assembly, accurate positioning,fracture surface reexamination and the like of the lagging jack are broken through, the lagging jack being lifted manually to be split is not needed in the tunnel construction process, and the whole set of construction process can be conducted through a mechanized manner, and is safer and more efficient. The mechanized construction method specifically comprises the construction steps that (1), thelagging jack is folded and hoisted; (2), the lagging jack intelligent mounting equipment is utilized to conduct sequential extending on the lagging jack, fabricated joints are automatically clamped,and a longitudinal connection device is mounted in advance; (3), clamping buckle lifting, accurate positioning and clamping fixing are conducted; (4) fracture surface reexamination is conducted; (5) core concrete is poured; and (6) the compactness is detected and a key part is reinforced.