An on-orbit drag-free control technique for spaceborne gravitational wave detection missions was discussed. Based on the analysis and design of a possible future geocentric orbit detection mission, the relative motion dynamics and coupling characteristics between the spacecraft and mass blocks of an on-orbit drag-free system with with two test masses were modeled. At the same time, the performance index and perturbation of the drag-free system in the mission were preliminarily analyzed, and a relative translational control law based on frequency domain H_∞ optimal control theory was designed. Numerical simulation results show that when the test masses of the two-test-mass on-orbit drag-free system are arranged according to the breathing angle of laser rangefinder, without a fixed tracking point strategy and without suspension control input along the non-sensitive axis, the spacecraft can achieve tracking of the reference point while meeting the frequency domain performance index of the system. At the same time, the time domain displacement of each test mass can be controlled to the micron level, thus obtaining the pure gravitational reference required by the mission.