Abstract: A new variable stiffness joint actuator is proposed by using a plate spring as elastic element and spiral discs as stiffness adjusting mechanism, and the stiffness regulation characteristics of the joint actuator are also discussed. A dynamic model considering the compliance and friction characteristics of the motor, synchronous belt and harmonic reducer is established. The correctness of the dynamic model is verified by position step and ball-hitting experiments. The research shows that the explosion of variable stiffness joint actuator is related to not only the stiffness but also the release time of stored energy, which provides a theoretical basis for improving the robot's dynamic characteristics when interacting with human.