The inter-frame differential method is the classical method for motion target detection, but it is difficult to detect a motion target by using this method when the motion target speed is as low as a certain value. Against at the limitation of its detectable target speed, a dynamic frame difference method is proposed based on the inter-frame difference method. Combining the characteristics of FPGA processing, the FPGA design and implementation of dynamic frame difference algorithm is carried out, and a new real-time motion target detection system is constructed. The system is capable of adaptively adjusting the frame spacing used for differential images according to the target motion, which is applicable to the detection of both faster and lower-speed moving targets, expanding the speed range of moving targets that can be detected by the system. Regional division of DDR3 ports solves the problem of multi-frame storage of images on FPGA with limited resources. Several sets of experimental results confirm the effectiveness and real-time performance of the dynamic frame difference method and its FPGA design module. Compared with other systems, this system has a clear advantage in detecting low-speed moving targets and can handle color video streams of 1080p@30Hz.