For the moving small dim targets in visible image sequences with low SNR and complex background, whose contained characters are simple and poor, they are extracted difficultly. This paper proposes a new method to detect and extract positions of small dim moving targets. According to the features of moving targets, in a very short time interval, the target trajectory is considered as a straight line approximately. Firstly, it makes use of threshold segmentation methods to extract the positions of targets in each frame, then building the motion line equations after the joint of multi-frame processing results. Finally, the position of small dim targets are detected out and extracted, and false targets are eliminated accurately. Hardware system was designed and the algorithm is implemented on hardware systems successfully. The experiment results show related functions of the system and extracting algorithm is feasible, the system is stable and has a strong process ability, which can effectively detect and extract small and dim target in complex background correctly.