A　space-stable　inertial　navigation　system　is　characterized　by　long-term　high　accuracy　and　a　complex　error　model.　It　is　now　in　pressing　need　to　calibrate　the　accelerometer　at　system　level　to　guarantee　high　precision.　The　main　challenge　is　how　to　simultaneously　calibrate　the　overall　9　accelerometer　errors　in　mooring　state.　We　propose　a　new　calibration　method　at　system　level　by　arousing　different　gravity　cones　of　the　platform　to　excite　all　the　accelerometer　errors.　Firstly,　observability　of　accelerometer　errors　are　surveyed　by　grids　means.　We　find　that　at　least　3　orientations　of　the　platform　are　required　to　calibrate　all　the　accelerometer　errors.　Secondly,　we　determine　the　optimal　combination　of　platform　orientations　by　minimizing　the　condition　number　of　observation　matrix.　Simulation　results　confirm　a　calibration　accuracy　at　1　x　10(-6)　g.　In　addition,　our　method　is　easy　to　implement　and　can　be　applied　in　a　relatively　low　cost.