This paper addresses the problem of designing an active disturbance rejection controller for a high accuracy drag-free satellite with cubic test mass. The uncertain model of the drag-free satellite is defined. The performance requirement imposed on the acceleration of the test mass is broken down into specification of drag-free and suspension loop because of the disturbance decoupling controller, which is based on the linear active disturbance rejection control technique. We derive two-degree internal model control structure of ADRC, which is used for robust stability verification. Search programs determine the parameters that satisfy system stability and the performance requirement. The design technique has shown to be robust to the perturbation of the system and good performance in disturbance suppressing. To check the design of the controller, an overall simulation is preformed, and the results confirmed that the controller is able to meet the system requirements.