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Valve-controlled electro-hydraulic technology is widely used in heavy-duty linear actuator, but its energy efficiency is low. The electro-mechanical actuator (EMA) has high energy efficiency, but its output force is small. In order to achieve high efficiency, high power-to-weight ratio, and high performance linear drive, a hydraulic–electric hybrid linear drive principle integrating hydraulic cylinder and EMA is proposed, and a prototype of electromechanical hydraulic hybrid actuator (EMHA) is designed and developed. In the hybrid drive system, EMA is responsible for actuator motion control and hydraulic cylinder is used for force control. In the study, an extended state observer (ESO) was first designed to estimate the external load torque of the EMA permanent magnet synchronous motor (PMSM) in real time. Combining the detected pressures, the load force of the EMHA can be estimated. Then, according to the estimated load force, the hydraulic system is controlled to make the hydraulic cylinder balance most of the actuator load force, maintaining the PMSM within the required load range. A sliding-mode controller is further designed to compensate the external load torque acting on the PMSM to ensure that the system has good motion control characteristics. The results show that under the same driving force and velocity, the power density of the EMHA is 154% higher than that of the EMA. In this article, the proposed system can achieve equivalent control performance as the valve-controlled system, while reducing energy consumption by 58%. [ABSTRACT FROM AUTHOR]