The solar panel based on harmonic drive is a complex rigid-flexible coupled system. This paper addresses the difficulty of high-precision motion control for solar panel based on harmonic drive, focusing on model order reduction methods and linear active disturbance rejection control methods. The dynamics of the harmonic reducer and solar panel are first analyzed, and mathematical models are established. Due to the high order of the controlled object leading to a complex model, we use the balanced truncation method for order reduction, ensuring that the low-order model obtained has similar characteristics to the full-order model within the mid-low frequency range. Then, according to the Linear Active Disturbance Rejection Control (LADRC), the controller is designed based on the relative order of the low-order model, and the information of the low-order model is integrated into the expanded state observer. Simulation results show that the active disturbance rej ection controller with model information has higher stability and dynamic performance compared to traditional low-order controllers. This method considers the system control bandwidth and effective relative order, adding model references in the observer, improving the system's anti-interference performance within the effective control bandwidth, reducing overshoot, and ensuring smoother operation of the solar panel. © 2023 IEEE.