The invention discloses an external-adjustable constant-pressure pre-tightening device for a motorized spindle. The external-adjustable constant-pressure pre-tightening device comprises a shell, a bearing support system and a pre-tightening adjustment system, wherein the bearing support system is consist of a bearing, an inner space ring, an outer space ring, a bearing block, a linear bearing, a fastening bolt and the like; the pre-tightening adjustment system is composed of a spring baffle plate, a bolt, a connecting plate and an adjusting bolt; the spring baffle plate is mounted in the shell, and the ear part of the spring baffle plate is connected to the connecting plate through the bolt; the adjusting bolt extends out of a rear cover of the motorized spindle through a center hole of the connecting plate. During working, the pre-tightening force applied to the bearing can be adjusted in real time by rotating the adjusting bolt. As an external-adjustable constant-pressure pre-tightening force mechanism is arranged between the shell of the motorized spindle and the bearing block, the axial preload of the bearing of the motorized spindle can be adjusted in real time according to different rotating speeds and different loads, the motorized spindle can be always in an optimal working condition, the internal heating phenomenon of the bearing is reduced, and the service life is prolonged. The external pre-tightening force is adjustable, so that the operation is convenient, and meanwhile an external complex device needed by the way that the pre-tightening force is adjusted through electromagnetism or hydraulic pressure is avoided.