The invention discloses a device for detecting a large-size shaft part. The device comprises a to-be-detected shaft support mechanism, a light transmission sensor group and a grating ruler, the to-be-detected shaft support mechanism comprises a support platform, a rotary platform, a free slideway, a tailstock and a guiderail, the rotary platform is provided with a first shifting top tip, the tailstock is provided with a second shifting top tip, the first shifting top tip and the second shifting top tip are used for fixing a to-be-detected shaft, the light transmission sensor group comprises an upper sensor and a lower sensor of an installation frame, the bottom part of the installation frame is connected with the guiderail through a slide block movably, the grating ruler and the guiderail are fixed on the support platform in a parallel manner, and a header of the grating ruler is arranged on the slide block. The device corrects the assembly error of the sensor, data of shaft diameter and shaft length of various segment positions of the to-be-detected shaft are measured, points not meeting the specifications on the to-be-detected shaft are marked, and the purposes of accurate measurement and wide scope of measurement are fulfilled.