This paper aims at providing a method to realize selfadaptive robotic hand, especially the under-actuation function among several fingers by springs. Considering its advantages of increasing stability of grasping and reducing difficulty of controlling, many studies on underactuated mechanisms have been carried out. Compared with traditional under-actuated mechanisms for single multi-joint finger, this paper proposed a novel multifinger under-actuated mechanism. By setting torsional springs around root joint shaft, the mechanism realizes that one motor drives root joints of multiple fingers. The main design principle is introduced through analysis. This mechanism can be used as root joints of index, middle, ring and little fingers. It also realizes passive adaptive grasp to objects of different shapes and sizes with a proper grasping force. It has high grasping stability and low control requirement.