The trajectory of the mass center of the cage is an important criterion for evaluating the motion stability of the cage. In order to accurately obtain the trajectory of the mass center of the cage, this paper proposed a new measurement method with line laser contour scanning, which dynamically monitors the change of the gap length between the cage and the inner ring in the radial plane in two mutually perpendicular directions. The amount of change in the gap length over time is equivalent to the radial displacement of the mass center of the cage, the displacement signal is filtered and processed to synthesize the trajectory of the mass center of the cage. This method does not need to change the bearing structure, is not limited by the driving speed and the space of the loading device, and can realize the trajectory of the mass center of the cage measurement under different working conditions. The test results under different rotational speeds verify the effectiveness of the contour scanning method, and it shows that the trajectory of the mass center of the cage tends to be stabilized when the inner ring speed increases from 1000 r/min to 3000 r/min.