Abstract: To address the challenges associated with bolt fastening procedures on angle-steel towers, a lightweight electric wrench with precise torque control was developed. A fusion algorithm combining the disturbance rejection controller and the sliding mode observer was proposed, achieving precise torque estimation and control of the electric wrench without the need for a torque sensor. Firstly, an active disturbance rejection controller was introduced into the torque and speed loops to compensate for the total disturbance and obtain the estimated angular acceleration. Then the estimated angular acceleration was utilized as the input of the second-order sliding mode observer to estimate the tightening torque, which effectively addresses the difficulty of accurately obtaining the derivative value of variables in the second-order sliding mode observer. Both simulation and experimental results show that compared with traditional PID controller and sliding mode controller, the proposed control method can achieve more precise torque estimation, coupled with reducing adjustment time and minimizing overshoot.