This paper proposes a GS/T mixed-sensitivity H_∞ controller design method based on spectrum specifications to tackle the multiple-degree-of-freedom control problem with high precision requirements of drag-free satellite in deep space mission. The method is explicit in physical meaning and can reduce the complexity in the design of drag-free control system. First, the high precision dynamic model of drag-free satellite with two test masses was derived, which shows the strong coupling characteristics of the loops. Next, selection matrices and input decoupling strategies were designed to divide the control system into single-input single-output (SISO) drag-free control loops, suspension control loops, and spacecraft attitude control loops, and closed loop feedback control strategies for each loop were further established. Then, combined with various scientific requirements and spectrum models of external disturbances and sensor noises, constrains for sensitivity functions and complementary sensitivity function of each loop were derived. With the help of these design criteria, weighting function matrices could be chosen quickly and accurately for the design of the drag-free and attitude control system in frequency domain. Simulation results proved the stability and capabilities of the proposed control system in resisting disturbances, which can achieve the ultra-quiet-stable requirement for drag-free control. Finally, on the basis of transfer function analysis, a method to further reduce acceleration noises was given.