Variable stiffness actuator (VSA) can imitate natural muscles in their compliance capbility, which can provide flexible adaptability for robots, improving the safety of robots interacting with the environment or human. This paper presents a new compact serial variable stiffness actuator ((SVSA-III)) with linear stiffness profile based on symmetrical variable lever arm mechanism. The stiffness motor is used to regulate the position of the pivot located on the Archimedean Spiral Relocation Mechanism (ASRM), so that the stiffness of the actuator can be adjusted (softening or hardening). By designing the lever length, the range of stiffness adjustment can change from 0.3Nm/degree to therotical infinity. Moreover, the continuous linear stiffness profile of the actuator can be customized by solving the transcendental equation of the relationship between the actuator stiffness and the rotation angle of the stiffness motor. SVSA-III has the advantages of compact structure, wide-range stiffness regulation, reduced control difficulty, and linear stiffness profile. Two experiments of step response and stiffness tracking have proved the high accuracy and fast response for both theoretical stiffness and position adjustment.