Two kinds of aerostatic bearing: two-row cylindrical bearings and closed-hole planar thrust bearing in the precision swing angle milling head spindle system were studied. To get the best film thickness and the corresponding maximum stiffness of the bearings in different throttling ways and under different Orifice diameter,the law of the bearing stiffness impacted by different structure parameters was obtained by using engineering design and calculation methods and the numerical simulation of the aerostatic bearing flow was carried out by FLUENT,which indicated that the change of the bearing stiffness and carrying capacity was caused by the change of eccentricity. Meanwhile the structural parameters of the aerostatic bearing were optimized. Results show that when the bearings are working under a small eccentricity ( ε < 0. 3 μm) ,its stiffness is a constant, and the carrying capacity increases with the increasing of eccentricity.