With the improvement of lithograph resolution, the depth of focus (DOF) of lithographic projection objective is decreasing. In order to take full advantages of the restricted DOF, focusing is commonly used to adjust the wafer onto the ideal focal plane. As the key point of focusing, research on focus detection becomes very popular. The present focus method is based on four-quadrant detector or array CCD to grab light signal, which carries the defocusing amount information of the wafer, and then process image on computer. This method is slow and cannot satisfy the real-time requirement for focusing. Therefore, a focus detection method, which wass based on line scan CCD for image grabbing and FPGA for image processing, was provided. This method can detect the defocusing amount in high-speed by utilizing the high-speed of line scan CCD and FPGA′s parallelism, combining with sub-pixel boundary detection algorithm based on polynomial interpolation. In order to compensate the defocusing amount, FPGA directly control the motor to drive the wafer stage up and down, making the focusing system a real-time closed loop. Due to reducing the computer links, this design has high-speed, high-resolution, low power consumption and low-cost characters.