Any kind of the under-actuated robotic finger traditionally has only one grasping mode, and each grasping mode is only suited for some situations but not for others. In this paper, a novel switchable under-actuated finger with multiple grasping modes, the SUA finger is proposed. Four typical grasping modes can be switched at any time freely. Two motors, multiple bevel gears, a flexible cable and two torsional springs are put forward to achieve this new function. A SUA finger is designed in detail, and the force analysis of the SUA finger mechanism is presented. The SUA finger that combines all the existing grasping modes has greater suitability, especially for grasping under unknown and complex environment.