The invention discloses an underground project assembly type lagging jack automatic connection joint and a mechanized construction method. An assembly type connecting device and a guide pressure bearing connecting device which are welded to two adjacent lagging jacks are included, and the assembly type connecting device and the guide pressure bearing connecting device are connected through a pin shaft rotating mechanism. Three rows of through holes are formed inside a connecting plate, guide stretchable devices are allocated in the holes, and the front ends of force transmission pins extend out of the holes through internal return springs. When the guide pressure bearing connecting device rotates around the pin shaft mechanism, the force transmission pins are extruded by a guide plate, thereturn springs are compressed, after a fixing connecting plate and the connecting plate are attached, the force transmission pins pop up to be combined with guide plate pin shaft holes in a clamped manner, joints are locked and connected together, and finally the joint strength and stability are improved through a bolt connecting mechanism. The joint is simple in structure, low in machining cost,simple in machining process, convenient and quick in welding and installing and wide in application scope.