The cross-coupling unit is the key component of the tri-axis vibration exciter. The static pressure bearing is applied in the cross-coupling unit to transmit vibration and guide motion for independent x－, y－, z-axial vibration. The transmission model of the cross-unit is modelled for stiffness selection according to the desired frequency range. A finite element method is introduced to solve the Reynolds equation of gas lubrication problem. Influence of air supply pressure and air film thickness on the stiffness of the air static pressure bearing is discussed. The stiffness of the static pressure bearing increases with the air supply pressure and there exists an extreme point of the film thickness where the static pressure bearing has the maximum stiffness at a given air supply pressure. On this basis, the parameter optimization of the cross-coupling unit is realized with a set of optimized air supply pressure and air film thickness. Experiments are carried out to and the results show that the optimized parameters can meet transmission characteristic requirement of the cross-coupling unit.