The invention relates to a collision detection method of a closed-loop mechanical arm system, in particular to a collision detection method of a closed-loop mechanical arm system based on a momentum observer, and belongs to the technical field of robots. According to the method, a motion sensor is arranged at the joint of the closed-loop mechanical arm system to measure joint motion information, asix-dimensional force or a moment sensor is arranged to measure binding force and binding moment at any at least one joint in each closed-loop, and collision identification and real-time detection ofcollision information of the closed-loop mechanical arm system are realized based on the momentum observer. The sensors required by the method are all arranged at joints, the assembly is simple, thereliability and the flexibility are high; according to the method, joint angular acceleration information or inversion of a high-dimensional time-varying mass characteristic matrix is not needed, thecollision information observation result is high in precision and the time delay is low; according to the method, the observer performance and the detection sensitivity can be adjusted by changing theobserver gain and the collision judgment threshold.