Gravity error influences three channels of airborne space stabilized inertial navigation system (SSINS). Gravity makes SSINS exponentially diverge. To overcome the divergence caused by the gravity, the altitude measured by aircraft altimeter device was used to calculate gravity compensation. The gravity calculation compensation method is feasible and correct. Finally, the theoretical analysis was validated by simulation. Results show that the introduction of external altitude information to calculate the gravity can effectively suppress the system divergence caused by gravity, and this gravity compensation method does not destroy the Schuler adjustment conditions of SSINS.