The invention provides a device for online measurement of a diameter of a center hole of a vehicle wheel. The device comprises a positioning base, a positioning sleeve and a displacement sensor; a probe column I and a probe column II are symmetrically installed in a positioning base hole I and a positioning base hole II of the positioning base; the positioning sleeve is installed on the positioning base and is provided with a positioning sleeve hole I, a positioning sleeve hole II and a central hole; the vehicle wheel is radially positioned and axially positioned by an outer circumferential surface of the positioning sleeve and a positioning surface of the positioning base respectively; measurement contacts are symmetrically installed on two sides of the displacement sensor; a probe pin I,the probe column I, a probe pin II and the probe column II which are symmetrically arranged transmit the displacement change quantity to the respective measurement contact; the probe column I and theprobe column II are in contact with the measurement contacts at respective positions; a spring I and a spring II are respectively sleeved on the probe column I and the probe column II; the probe pinI and the probe pin II are respectively screwed on the probe column I and the probe column II through screw joint and respectively pass through the positioning sleeve hole I and the positioning sleevehole II in the positioning sleeve.