To reduce the influence of gravity disturbance torque on the balance and simulation accuracy of the three axis airbearing testbed, the calculation method for unbalance torque raise by gravity gradient of airbearing testbed was presented. According to the existing slightly difference by gravity on each particle of the platform, the computational model of unbalanced torque caused by gravity gradient was established and the influence of gravity gradient unbalance torque on the equilibrium of airbearing testbed was analyzed. The simulation results show that the gravity gradient unbalanced torque is the sine function as two times tilt angle of the platform. With the mass center approaching to rotation center, the gravity gradient unbalance torque is gradually becoming the main factor affecting the airbearing platform balance.