A road test on semi-trailers is carried out, and accelerations of some characteristic points on the braking system,axles,and truck body is measured,also brake pressure and noise around the support frame is acquired.The measured data was analyzed to determine the causes of the brake noise, and the mechanism of the noise of the drum brake of semi-trailers during low-speed braking was investigated. The following conclusions are obtained: (1) Brake noise of the drum brake of the semi-trailer at low-frequency is generated from vibrations of the brake shoes, axle, and body, and the vibration frequency is close to 2nd natural frequency of the axle. (2) Brake noise is generated from stick-slip motion between the brake shoes and the brake drum, where the relative motion between the brake drum and the brake shoes is changed alternately with sliding and sticking, resulting in sudden changes in acceleration and shock vibration. A multi-body dynamic model of the semi-trailer is established for analyzing vibrations causing noise and the influencing parameters. In the model, the elastic deformation of components, such as brake drums, brake shoes, axles, and leaf springs during the braking process, is considered. The model is validated by comparing calculated data with experiment data.The simulation shows that there is a heavy stick-slip vibration between the brake drum and brake shoes, which is transmitted to the axle through the brake shoes, and then to the body through the leaf spring. As the speed of the semi-trailer increases, the stick-slip frequency between the frictional pairs increases. When the stick-slip frequency is close to the natural frequency of the axle, it resonance.