Since animals can adjust the stiffness of their bodies and/or appendages to adapt to changing environments and internal states and tasks, a robotic leg with changeable stiffness would be of assistance in developing mobile terrestrial robots. In this article, we present an improved model of a robotic leg that can alter its stiffness in real time by changing the characteristics of magnetorheological fluid. This particular robotic leg has a changeable stiffness module, which consists of a rotating magnetorheological fluid damper, a torsional spring, and a linear spring. The rotary magnetorheological fluid damper is used to control the deformation of a torsional spring, which alters the stiffness of the leg. To describe the mechanical features of the leg, a simplified mechanical model was built and then various experiments were carried out to verify the changeable stiffness characteristics. The simulation experiments of dual-leg locomotion were carried out to investigate the locomotive performance, including walking speed, maximum torque of motor, height variants, and mechanical cost of transport. These results demonstrate the adaptability and advantages of this changeable stiffness robotic leg and also indicated that developing a terrestrial robot which can adapt to various complex environments and tasks would be a worthwhile exercise.