To satisfy the motion control of numerical control machine and robot, this paper introduces an approach to implement 4-axis motion controller based on field programmable gate array (FPGA). Starting with introduction to existing excellent 4-axis motion controller MCX314, this paper describes the fundamental structure of the FPGA-based 4-axis motion controller. As the one of main function modules in controller, the implementation of 2-axis circular interpolation module in FPGA is the main content to be discussed in the paper. Employing a novel analogy digital differential analyzer (DDA), the interpolation module avoids plentiful complex on-the-motion computation with skillful combination with the hardware structure of FPGA; hence the real-time performance and precision are enormously improved. In the paper, the principle of algorithm and approach of hardware implementation are discussed in detail; with introduction to the structure of the circular interpolation module, a testing platform is developed to evaluate the performance of the module. The testing experiment indicates that the performance of controller is excellent, and the implementation approach will be valuable for domestic research and development about the motion controller.