The series elastic actuator (SEA) has the advantages of flexibility, safety and high energy efficiency. Compared with proprioceptive actuator, it’s more suitable for heavy load legged robots. However, a SEA often has a long volume in the axial direction, which will bring trouble to the design of legged robots’ structure. In this paper, a new practical hollow shaft SEA with an innovative elastic element in a thin-walled cylinder shape is proposed. Due to the great compact design and the novel shape of the elastic element, the proposed SEA is only 66 mm long axially with a peak torque of 40 Nm. To optimize the shape of the elastic element, the authors combine orthogonal experiments with finite element analysis, and also propose a variable stiffness scheme. At the same time, the dynamic model is further analyzed, and an online parameters identification method is proposed, which is helpful to improve the control capability.