In order to address the problem that disturbance compensation effect of traditional active disturbance rejection controller(ADRC) decreased rapidly with the increase of disturbance frequency, a new ADRC method based on modified linear extended state observer(ESO) was proposed in this paper. Firstly, the coarse tracking system was analyzed, simplified and identified, and then the disturbance observation theory of ESO was derived and demonstrated, and its disadvantages were pointed out theoretically. According to that, an improved linear ESO was proposed and its advantages were illustrated by theory and simulations. Finally, the ADRC of coarse tracking system was realized with the improved linear ESO and PID adjuster. The experimental results show that as for the external position disturbance with the magnitude of 1° and frequency of 0.5-2.5 Hz, the disturbance isolation degree declines significantly with the increase of disturbance frequency adopting the traditional ADRC. While the disturbance isolation degree improves at least 4.416 dB adopting the improved ADRC in this paper, and the disturbance isolation degree increases stably with the increase of disturbance frequency, which is almost the same at 2.5 Hz and 0.5 Hz. What′s more, the proposed method is robust and the change within 20% of the controlled object is allowed. In conclusion, theoretical analysis, simulation analysis and the physics experiment prove this method is effective, and it has some reference value for similar photoelectric tracking system.