Abstract: A permeable ditch underwater water purification apparatus purifies source water using the principle of self-purification of water. This apparatus includes impermeable ditch walls on a tapered bottom. The apparatus also has an open top side. A granular filter layer is filled in the ditch body. Collector (counter-rinsing) pipes are provided at the bottom of the granular filter layer, and a filter protecting layer is provided on the surface of the granular filter layer. This apparatus can maintain the thickness of the granular filter layer all the year round. When blockage of the filter layer occurs, water from another ditch is pumped, and the remaining impurities in the granular filter layer can be forced away by water pumped into the collector (counter-rinsing) pipe. After sterilization by adding an appropriate amount of chlorine into the purified water, the quality of water will meet the standards for drinking water in effect in China, and the purified water may be supplied directly for use by the users.