An innovative wearable upper-limb power-assist exoskeleton system (UPES) was designed for la-borers to improve work efficiency and reduce the risk of musculoskeletal disorders. This novel wearable UPES consists of four joints, each comprising a single actuated pneumatic muscle actu-ator (PMA) and a torsion spring module driven via a steel cable. Unlike most single-joint applica-tions, where dual-PMAs are driven by antagonism, this design aims to combine a torsion spring module with a single-PMA via a steel cable for a 1-degree of freedom (1-DOF) joint controlled by a proportional-pressure regulator. The proposed four driving degrees of freedom wearable UPES is suitable for power assistance in work and characterizes a simple structure, safety, and compliance with the motion of an upper limb. However, due to the hysteresis, time-varying characteristics of the PMA, and non-linear movement between joint flexion and extension, the model parameters are difficult to identify accurately, resulting in unmeasurable uncertainties and disturbances of the wearable UPES. To address this issue, we propose an improved proxy-based sliding mode con-troller integrated with a linear extended state observer (IPSMC-LESO) to achieve accurate pow-er-assisted control for the upper limb and ensure safe interaction between the UPES and the wearer. This control method can slow the underdamped dynamic recovery motion to tend the target trajectory without overshoots from large tracking errors that result in actuator saturation, and without deteriorating the power assist effect during regular operation. The experimental re-sults show that IPSMC-LESO can effectively control a 4-DOF wearable UPES, observe the un-known states and total disturbance online of the system, and adapt to the external environment and load changes to improve system control performance. The results prove that the joint torsion spring module combining the single-PMA can reduce the number of PMAs and proportion-al-pressure regulators by half and obtain a control response similar to that of the dual-PMA structure.