Machine tool is the foundation of manufacture,then, five-axis CNC machine is the sophisticated equipment of CNC machine. During the practical work,there is an thermal deformation in a CNC machine because of many kinds of heat sources . This deformation affects the relative displacement between the work piece and the cutter, caused the manufacturing errors, and influences the machining accuracy of the part. So is of great importance to research the thermal characteristics. In this paper, thermal characteristics of CNC milling machine are simulated by analyzing the thermal boundary condition and using the finite element software-ANASYS Workbench. Get the temperature field cloud picture of A/C axes bi-rotary mill head of five-axis CNC machine. Weakness of Thermal Characteristics will be found out. This research provides the theoretical fundamentals for improving the accuracy of the mill head.