To solve the problem that the passive millimeter wave technology has not enough capability to detect high-speed moving targets, a passive millimeter wave detection system is built by using a direct detection millimeter wave radiometer based on a Monolithic Microwave Integrated Circuit (MMIC). The data of a high-speed moving target are measured under different conditions in experiment. On the basis of the analysis of the characteristics of the test signals, it is pointed out that the peak of the first pulse and the spacing between two pulse peaks can reflect the effective motion characteristics of the target. A method for calculating the motion speed of the target is proposed. The results show that the method has a certain precision and can be used in the passive millimeter wave detection system to determine the parameters of the moving target.