High-precision displacement measurement can help realize high-precision microgravity， which can further sever a variety of space science payloads for research missions. In this study， we devised a dual-frequency interferometer based on three sets of orthogonal symmetric cube corner retroreflectors （CCRs） for measuring the six-degree-of-freedom （6DOF） pose of the spatial inertial test mass （TM） through heterodyne detection. We first established an optical model of the actual CCR， which considers the optical path difference caused by the motion of the TM， and derived the analytical relationship between the pose of the TM and the measured optical path change. Then， the attitude angles and displacements of the TM were calculated using the method of numerical analysis， which affords remarkably higher accuracy than does the traditional small-angle approximation method. Furthermore， the performance of our system was estimated using space in-orbit data and random data. The simulation results show that the displacement error was less than 0.02 fm even when the TM vibrated significantly， indicating that our method affords high accuracy and good adaptability. In addition， the error sources of the system were analyzed. The measurement errors for the attitude angle and displacement are less than 0.017° and 10 nm， respectively， when the angular installation error is below 5 mrad， the distance installation error is less than 10 μm， and the parallelism is less than 2 mrad. The proposed 6DOF measurement and calculation method can also be used in various precision machining and detection applications.