A special-purpose testing device for bearing preload was developed.The relationship between preload and system natural frequency was researched on this test rig.The test data of bearing preload versus system natural frequency were analyzed.The results show that the preload test rig can reflect the bearing's preload characteristic to a certain extent.The system natural frequency increases with the rising bearing preload.At the same bearing preload,the system natural frequency increases with the increase of bearing's supporting span.The precision of the fitted relationship of preload-natural frequency-bearing span is higher.