Abstract: In order to improve the controllability and safety of the robot in complex working conditions, a new flexible variable-stiffness joint actuator（VSJA） is proposed based on the leaf spring and variable fulcrum lever structure. The actuator realizes the stiffness change through the elliptical groove gear transmission mechanism, which has compact design and large stiffness range, and can be adjusted continuously. Based on the flexural deformation theory of cantilever beam and Taylor formula, the explicit stiffness expression of the variable-stiffness joint is derived, which can be used to determine the stiffness of the actuator. The stiffness variation of VSJA is analyzed by Matlab and the rationality of the theoretical model is verified by Adams. Finally, an experimental platform is built to verify the feasibility of VSJA. The experimental results show that the theoretical stiffness of VSJA is consistent with the experimental results, and it has good position tracking performance and can meet the variable-stiffness requirements of the robot joint.