As the effect of atmospheric particles scattering, the video image captured by outdoor surveillance system has low contrast and brightness, which directly affects the application value of the system. The traditional defogging technology is mostly studied by software for the defogging algorithms of the single frame image. Moreover, the algorithms have large computation and high time complexity. Then, the defogging technology of video image based on Digital Signal Processing (DSP) has the problem of complex peripheral circuit. It can’t be realized in real-time processing, and it’s hard to debug and upgrade. In this paper, with the improved dark channel prior algorithm, we propose a kind of defogging technology of video image based on Field Programmable Gate Array (FPGA). Compared to the traditional defogging methods, the video image with high resolution can be processed in real-time. Furthermore, the function modules of the system have been designed by hardware description language. At last, the results show that the defogging system based on FPGA can process the video image with minimum resolution of 640×480 in real-time. After defogging, the brightness and contrast of video image are improved effectively. Therefore, the defogging technology proposed in the paper has a great variety of applications including aviation, forest fire prevention, national security and other important surveillance.