In order to meet the urgent requirements for joint flexibility and variable stiffness characteristics in the dynamic motions of legged robot, such as running and jumping, inspired by the flexible features and activepassive stiffness adjustment mechanism of biological joints, a new type of variable stiffness bionic flexible joint based on cam mechanism was proposed innovatively. Based on the analysis of the joint stiffness characteristics, a joint integral stiffness model was constructed. Then, the joint structural parameters affecting its stiffness characteristics were designed optimally and systematically, and a compact highly integrated joint prototype was developed. The joint prototype performance experiment results show that the variable stiffness bionic flexible joint based on cam mechanism has desired joint output torque and stiffness adjustment range. Through the activepassive fusion control of joint intrinsic and dynamic stiffness characteristics, the nonlinear and accurate adjustment of joint instantaneous stiffness is achieved dynamically, which meets the requirements for joint flexibility and stiffness of robot dynamic movement.