Auxiliary bearings (ABs) are important backup and safety components in active magnetic bearing (AMB) systems. The performance of ABs significantly affects the reliability, safety, and serviceability of the AMB system and the rotating equipment. Research on the dynamic behavior during the touchdown process is crucial for analyzing the severity of the touchdown and the behavior is closely related to the performance of the AB. In this paper, a friction torque coefficient is introduced to evaluate the performance of AB and the touchdown process is simulated and evaluated with various values of this coefficient with a detailed rotor-AB model of a vertical AMB system. The results show that large friction torque coefficient will result in undesired backward whirl of the rotor and catastrophic large load on the AB. Based on these results, this paper suggests that large friction torque coefficient of AB should be avoided in practice.