The transformation of tool position and orientation has impact on processing performance of the system when processing complex surface. Five-axis NC machine center was taken for example in this article. Semi-analytical calculation method was taken for stiffness field of system. Based on virtual-work principle, the composite stiffness field modeling of system was built by methods of Jacobian matrix, point transformation matrix and finite element, etc. For the key point of workpiece surface which is needed analyzed, 3D space force ellipsoid was introduced, composite stiffness field was modeled and composite stiffness performance of system was analyzed by force ellipsoid with different tool position and orientation. Guiding conclusion was obtained for following works by analyzing influence of tool position and orientation on stiffness performance.