An automatic pipe dust cap assembly for water pipes, characterized by including a tube feeding system, a cap assembly system, and a control system. The tube feeding system comprises a frame and multiple tube feeding disc wheels. The cap assembly system is positioned on the side of the tube feeding disc wheel mounted on the frame, and includes multiple radial positioning components for water pipes and cap assembly components. The cap assembly components consist of a cap component drive mechanism and a cap component positioning mechanism. The tube feeding system and the cap assembly system are controlled by the control system. The tube feeding disc wheels rotate the water pipe to the position for cap assembly. The radial positioning components of the water pipe limit the radial direction of the water pipe. The cap component drive mechanism and the cap component positioning mechanism transmit and position the cap, fitting it onto the end of the water pipe.