The invention relates to the field of agricultural irrigation, in particular to an automatic irrigation system for farmland ditches. The automatic irrigation system comprises a farmland, a ditch, a pipeline mechanism, two supporting mechanisms, four stabilizing mechanisms, a plurality of filtering mechanisms, a plurality of irrigation mechanisms, a liquid storage mechanism and a recycling mechanism, and the ditch is used for installing irrigation equipment; the pipeline mechanism is used for conveying irrigated liquid; the two supporting mechanisms are used for supporting and installing a pipeline mechanism, and the two supporting mechanisms are the same in structure; the four stabilizing mechanisms are used for stabilizing each supporting mechanism, and the structures of the four stabilizing mechanisms are the same; the multiple filtering mechanisms are used for filtering liquid in the ditch, and the structures of the multiple filtering mechanisms are the same; the structures of the multiple irrigation mechanisms are the same; and the recycling mechanism is used for recycling the liquid in the ditch. The problems that traditional ditch irrigation equipment is inconvenient to install, the irrigation range is not wide, and the liquid in the ditch cannot be recycled are solved, the irrigation range of the farmland is widened, resources are saved, and agricultural personnel can conveniently install the irrigation equipment.