With its high torque ratio and stable low-speed operation, the segmented arc permanent magnet synchronous motor (SAPMSM) provides high-performance drive technology support for large-aperture astronomical telescope observations. Improving the motor’s performance is challenging due to various internal and external interferences during its operation, such as parameter distortion, harmonics, etc. To this end, this paper proposes an integral sliding mode controller based on a new reaching law and a hybrid control strategy that combines an expanded state observer and a load observer, aiming to optimize the traditional sliding mode control and enhance the system’s anti-interference ability. The traditional reaching law has complicated parameters and cannot suppress chattering well. The new reaching law simplifies the parameters and effectively overcomes the system chattering. Second, an expanded state observer is used to estimate the feedback speed. Then, the q-axis current information and the estimated speed data are combined as the input of the load torque observer. This further improves the load observation performance and converts the load observation value into current for pre-processing. Feedback compensation is used to improve the motor’s anti-interference performance. Simulation and experimental results show that the proposed dual observer method can effectively observe the motor's speed and load, significantly enhancing the motor’s ability to resist load disturbances. At the same time, the new sliding mode speed controller reduces the motor speed overshoot and suppresses the buffeting of the sliding mode to a certain extent, providing theoretical and experimental support for arc motors in high-precision observation applications of large-aperture astronomical telescopes.