Abstract: This paper presents a self-centring bridge pier system connected by unbonded post-tensioned tendons and mild steel which can minimize the post-earthquake residual deformation and displays stable energy dissipate capacity. The basic mechanical concept of a self-centring system is explained. Referring to the quasi-static test, a simplified mechanical model and a finite element model is herein illustrated, considering flexural deformation, unbonded length of mild steel and strain penetration due to strain differences between the bar and concrete within the connection member. To solve the stress of concrete, two alternative methods, the equivalent rectangular stress map method and plane-section assumption method, are also critically discussed for better understanding of the principle of a self-centring system. Moreover, to provide a guide for structure design, the author indicates the intrinsic characteristic of the stiffness of a self-centring bridge pier which could result in vibration isolation.