<p>Series elastic actuators (SEAs) offer some benefits, such as shock absorption, power amplification, and buffering capabilities. Among them, hydraulic SEAs have added advantages related to high power density, thereby improving the performance of robotic joints. This paper proposes an innovative bi-directional flexibility single-spring mechanism, serving as a foundational element of a linear single-spring hydraulic SEA (SH-SEA). A high-performance admittance controller for the linear SH-SEA is also developed. The advantages of the new single-spring mechanism are thoroughly investigated. After comparison, the initial size of the spring in the single-spring mechanism is less than half the initial size of all springs in the conventional double-spring mechanism, or the energy storage capacity of the single-spring mechanism is more than twice that of the double-spring mechanism. So, the SH-SEA is inherently compact and has better energy efficiency. The proposed admittance controller self-optimizes compensation parameters for the hydraulic cylinder’s asymmetry and the expected position’s amplitude attenuation. It also compensates for time delay of the system. Experimental results demonstrate the effectiveness of the proposed admittance controller in accurately tracking both the position and stiffness of the linear SH-SEA.</p>