The invention belongs to the field of machining technology, and to device for testing a circular shaft. The device comprises a bracket, a supporting block, a dial gauge, a motor, a gear and a grabbingmechanism. The middle part of the bracket is provided with a conveying mechanism for conveying the circular shaft. The lower part of the bracket is provided with a collecting unit for collecting thecircular shaft. The upper part of the bracket is provided with an arc-shaped chute. The radius of the chute is the distance between two hydraulic cylinders, wherein the lower end of one hydraulic cylinder is slidably arranged in the chute. One end of the chute is provided with an electromagnet. The lower end of the hydraulic cylinder in the chute is provided with a permanent magnet which is attracted with the electromagnet. A supporting spring is arranged between the permanent magnet and the electromagnet. The electromagnet is provided with an on switch and an off switch which are electricallyconnected with the electromagnet. The off switch is arranged on the hydraulic cylinder and is a gravity induction switch. Compared with the prior art, the device which tests coaxiality of the circular shaft has advantages of low labor intensity, high efficiency and high accuracy.