The invention relates to a mainshaft prepressing device, in particular to a high-speed mainshaft bearing constant-pressure pre-loading device which comprises a mainshaft box, a first bearing and a second bearing arranged at the front end and the rear end of the mainshaft box, and a mainshaft arranged in the first bearing and the second bearing, wherein the first bearing is fixed on the mainshaft box, an inner sleeve is arranged between the first bearing and the second bearing, the second bearing is arranged in a sliding sleeve in an interference manner, the sliding sleeve is in clearance fit with the mainshaft box, and one side of the sliding sleeve is provided with a pre-loading component. Compared with the prior art, the invention adopts contant-pressure pre-loading, the second bearing is arranged in the sliding sleeve in an interference manner the sliding sleeve is in clearance fit with the mainshaft box, and one side of the sliding sleeve is provided with the pre-loading component, therefore, axial movement can be realized, deformation of a mainshaft system caused by temperature rise under high-speed running can be compensated, stuck phenomenon of the bearing can be avoided, and the work stability of the mainshaft system can be effectively improved.