The invention relates to a buried irrigation loop system and an irrigation method for cotton fields. The system comprises a main pipe, submain pipes, branch pipes and capillary pipes, wherein the branch pipes and the capillary pipes are arranged in I-shaped structures, each I-shaped structure is composed of two branch pipes and multiple capillary pipes arranged between the two branch pipes, and the I-shaped structures form a field pipe network; in each I-shaped structure, a region valve, an exhaust valve and a pressure gauge are arranged at the head end of each branch pipe connected with the corresponding submain pipe, and a pressure gauge, an exhaust valve and a scouring valve are arranged at the tail end; a pressure gauge, an exhaust valve and a scouring valve are arranged at a diagonal corner to a water inlet of the I-shaped structure, and an exhaust valve is arranged at the other end. The buried irrigation loop system is simple in structure, wide in applicability, convenient to use, easy to grasp and capable of solving the problems of uneven pressure on the head portion and the tail portion and easily blocking of the capillary pipes through common drip irrigation under film.