An automatic pipe dust cap assembly device for water pipes, characterized by including a tube selection system, a cap assembly system, and a control system. The tube selection system comprises a frame and multiple tube selection wheel disks. The cap assembly system is positioned on the side of the tube selection wheel disk 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 transmission mechanism and a cap component positioning mechanism. The tube selection system and the cap assembly system are controlled by the control system. The tube selection wheel disks rotate the water pipe to the position for cap assembly. The radial positioning components for water pipes limit the radial position of the water pipe. The cap component transmission mechanism and the cap component positioning mechanism transmit and position the cap, which is then assembled onto the end of the water pipe.