To realize precise tracking of test mass in a drag-free satellite, the relative motion between the test mass and the outer satellite is derived and the main disturbance sources in the relative dynamics are analyzed. Based on the analysis, an adaptive controller for the displacement mode of drag-free satellites with single test mass is designed and it is applicable for unknown but constant or slow-varying satellite mass and spatial disturbances. Under the assumption that both the satellite mass and the disturbances are unknown constants, the controller can guarantee global asymptotic convergence of the tracking errors. Simulation results are given to show the efficiency of the controller.