Abstract: The flexibility of robot joints plays an important role in the human-robot collaboration process. However, the fixed flexible joint cannot meet the demands for the dynamic variation of human-robot collaboration. Thus, it is necessary for the robot joint actuator to be capable of stiffness tuning. A stiffness control method that can be applied to a series elastic actuator with fixed stiffness and in which Archimedes spiral spring is adopted as the flexible element of robot joint is proposed. According to the definition of joint stiffness, the measured output angle of spring can be used to calculate the input angle of spring, so that the equivalent stiffness of robot joint actuator can be adjusted into the expected value. This method regards the motor position controller as the inner loop and the joint stiffness controller as the outer loop, which simplifies the design of controllers and enables the decoupling control. The designed stiffness controller is theoretically analyzed. Finally, a stiffness adjustment experiment was carried out on the experiment platform with the self-designed single DOF thin-type series elastic actuator for cases of two-way step of stiffness, zero stiffness, and stiffness with sinusoidal variation. The experiment results show that the equivalent stiffness of joint can accurately trace the expected value, which verifies the validity of the proposed method.