Credence service is distinct from other types of services because of its professional and/or pure nature. Its formation mechanism of customer loyalty is also unique. The empirical analysis based on medical service shows that in trust service, customer trust plays a key role in the formation mechanism of credence service loyalty and forms the logic chain from service quality, to customer trust, to perceived value, to customer satisfaction and then to customer loyalty. The interaction quality and outcome quality of service can affect perceived value and customer satisfaction through trust, and ultimately affect customer loyalty. However, service environment quality has no significant effect on the formation mechanism of credence service loyalty. Therefore, the research on customer formation mechanism needs to distinguish the type of products and service, the organizations providing trust service should emphasize and strengthen the cultivation of the interactive capacity of service personnel and customers, promote the interactive quality of service between service persons and customers and really ensure and boost service outcome quality in order to uphold customer trust and customer loyalty.