To improve the machining speed and precision of mechanical parts with complex shapes by Computer Numerical Control (CNC) machining, a B spline interpolation method for 5-axis CNC machine tools is proposed. By taking the Open and Modular Architecture Controller (OMAC) as a reference, a motion controller with B-spline interpolation function is developed.The controller divides the controlling tasks in accordance with real-time request, executes the man-machine interactive, code analysis and parameter mapping in an offline environment and runs the interpolation operation, discrete logic control and inverse kinematic transformation in a real-time thread.Therefore,the real-time performance of the controller is well realized by methods above. To simplify the procedure of NC program, the machining information received by the controller are vectors in a part coordinate system.Furthermore, the controller can offer a good generality by developing Inverse Kinematic Transformation Module (IKTM) units suitable for various 5-axis machine tools and designing the parameters of machine tools as the customizable parameters.The mapping relation between the orientation and the position parameters in the NC program is established by the controller. As a result, the motions of linear axes and motion of rotary axis meet the machining requirement well and good machining accuracy is obtained. In a machining process, when the NC program capacity for B spline interpolation is 15% that of the NC program capacity for linear interpolation, its interpolation error is 0.45% that of the linear one. The interpolation accuracy of the controller is 0.68.These results show that the developed B spline interpolation controller can well meet demands of NC machining and has a good controlling performance.