China Aero Geophysical Survey and Remote Sensing Center successfully developed AGS-863 Three Axis Airborne Magnetic Gradiometer with self-owned intellectual property rights, and carried out gradient measurements of aeromagnetic survey in Inner Mongolia recently. To analyze the key indicators of gradiometer and corroborate instrument performance, the authors designed the testing method for key indicators. It has been proved that the aeromagnetic gradiometer is stable and reliable during survey. The test result shows that the system has excellent synchronism, consistency, long-term stability and compensation precision.