The mathematic model of the spindle pose and workpiece flatness for the ultraprecision flymilling is established to investigate influence on workpiece flatness caused by the spindle pose tilt on ultraprecision flymilling(UFM). The numerical correspondence of workpiece flatness and spindle pose could be determined according to the simulation results. The rapid measurement and adjustment method for spindle pose in the workspace were presented base on established model between the workpiece flatness and spindle pose. The experiment was carried out using an 50 mmdiameter workpiece of Al6160 material，after the spindle pose was measured and adjusted, the flatness could be improved to be 086 μm from 228 μm. The experiment result indicates that spindle pose error can be determined by onmachine measurement based on the proposed mathematic model. The flymilling flatness can be improved to be submicron level effectively.