Farmland closed drainage system and construction method

The invention relates to the technical field of farmland drainage, and provides a farmland closed drainage system. The farmland closed drainage system comprises closed conduits, and the closed conduits are arranged in farmland at equal intervals. Drainage sand wells are further included and are made from coarse gravel and fine gravel. The drainage sand wells are located on the upper portions of the closed conduits, and arranged at equal intervals in the axis direction of the closed conduits. The bottoms of the drainage sand wells and filter materials around the closed conduits are connected as a whole. The invention further provides a construction method of the farmland closed drainage system. The construction method comprises the steps that pile positions of the drainage sand wells are determined; surveying and setting out are carried out; drainage sand well foundation pits are dug; the drainage sand wells are manufactured; the tops of the drainage sand wells are backfilled with farmland mellow soil. The farmland closed drainage system and the construction method of the farmland closed drainage system have the beneficial effects that the drainage sand wells partially penetrate through an aquitard, the drainage sand wells are high in water permeability, and the hydraulic connection of the soil on the upper portion and the lower portion of the aquitard is built; a composite drainage network is formed by the drainage sand wells and the closed conduits; compared with a traditional closed drainage system, drainage efficiency is higher.