The invention relates to a farmland water-saving irrigation system based on a river and belongs to the technical field of farmland irrigation. The farmland water-saving irrigation system comprises a controller, irrigation equipment and a submersible pump which is arranged in the river and is connected with the irrigation equipment; the irrigation system further comprises a primary purification unit arranged in the river, a secondary purification unit located between the submersible pump and the irrigation equipment and an air spraying unit, wherein the primary purification unit comprises a water purification box, an opening is formed on the upper end of the water purification box and a coarse filtration screen is arranged at the opening position; the secondary purification unit comprises awater conveying disc provided with one group of flow splitting cavities; one group of fine filtration screens are arranged on the water conveying disc in a manner of corresponding to the flow splitting cavities; the submersible pump is arranged in the water purification box and is connected with water inlets of the flow splitting cavities through pipelines; water outlets of the flow splitting cavities are connected with the irrigation equipment; the air spraying unit is provided with an air spraying pipe with an air spraying opening located in the water purification box. According to the irrigation system provided by the invention, pumped river water is purified and then is used for irrigating, so that the irrigation system can be applicable to relatively water-saving irrigation manners including sprinkling irrigation, drip irrigation and the like.