This paper proposed a design idea of a novel under-actuated finger mechanism, and designed the finger mechanism. The finger has no actuator in itself, is only driven by the other finger joints and object grasped. The finger is similar to a human finger and can be easily arranged in series to realize a finger with super under-actuation and high integration. It can be mounted in humanoid robot hand to make the hand obtain more DOFs with less actuators, and good grasping function of shape adaptation, decrease the requirement of control system. This paper analyzed the relationship between the grasping force of the finger and its design parameters, proposed the design principle of structure optimization of the finger. Based on the finger, a multi-fingered humanoid robot hand: TH-2 Hand has been designed. TH-2 Hand has many excellent features: high personification, super under-actuation and be very compact, easy to real-time control, small volume, light in weight, strong grasping function, etc.