The invention discloses an online detection device for the diameter and the radial runout of a rotary body part, which comprises a conveyor belt, a first push mechanism, a second push mechanism and a measurement and control system, and is characterized in that the tail end of the conveyor belt is provided with a baffle plate, the first push mechanism is arranged at one side of the tail end of the conveyor belt, the first push mechanism pushes a rotary body part on the conveyor belt to a measuring mechanism, the rotary body part is pushed to a turnover mechanism through the second push mechanism after being measured on the measuring mechanism, one side of the turnover mechanism is provided with a qualified part storage basket, the other side of the turnover mechanism is provided with an unqualified part storage basket, qualified parts are sent to the qualified part storage basket by the turnover mechanism, and unqualified parts are sent to the unqualified part storage basket by the turnover mechanism. The online detection device realizes relative motions of the rotary body part and a digital micrometer through a rising and rotating mechanism. Meanwhile, the digital micrometer measures the circular surface diameter and an eccentricity value at different moments, thereby being easy to calculate the multi-point diameter and the radial runout of the part.