Abstract: To study the fretting behaviors of locking device for magnetically suspended flywheel under different locking forces, the swept-sine vibration and random vibration were carried out to simulate the vibration environment during launch. The fretting displacements between stator and rotor measured by eddy current displacement sensor indicate the composite fretting regime including tangential and radial fretting in locking surface. After tests, the surfaces of three contacts including 1Cr18Ni9Ti/GCr15, nitrided 1Cr18Ni9Ti/GCr15 and GCr15/GCr15 were analyzed using scanning electronic microscopy. The results show that with the increase of locking force from 200 N to 300 N, the maximum fretting displacement decreased from 60 μm to 8 μm and its corresponding amplitude decreased from 25 μm to 4 μm. This results in tangential fretting transition from slip regime to partial slip regime and reduced the fretting damage. In addition, the micro-slip and radial fretting damage can be avoided by increasing contact stiffness and choosing similar contact bodies. The two fretting resistances can inhibit composite fretting damage in locking surface.