To improve the sensitivity of collision detection when cobot runs at high speed, a collision detection method based on momentum observer and dual encoders was proposed. An external torque observer was constructed according to the dynamics model of the robot based on generalized momentum. The external torque was estimated by the real-time position deviation of dual encoders and the equivalent flexibility characteristics of the robot joint transmission structure. A reaction strategy was designed to realize the robot to stop quickly and could be pushed manually after collision. The simulation and experiment results show the proposed algorithm has shorter response time and smaller maximum collision force than the traditional algorithm based on momentum deviation observer when the robot runs at high speed, which improves the sensitivity of collision detection and the safety protection effect.