The invention belongs to the field of actual large-span bridge structure stay cable structure damage diagnosis, and particularly relates to a BOTDA technology-based large-span bridge stay cable structure damage diagnosis method. The method comprises steps of introducing BOTDA technology into stay cable structure damage diagnosis, and obtaining strain monitoring data of multiple sections of a mainbeam in real time through distributed sensing optical fibers; performing numerical simulation by utilizing the finite element model of the cable-stayed bridge, and constructing a main beam strain-mainbeam vertical deflection and main beam vertical deflection-stay cable force relationship model; constructing a stay cable damage diagnosis factor and a threshold value by adopting the multi-section strain monitoring data of the bridge girder structure in the healthy state, and judging whether the stay cable is damaged or not by comparing whether the numerical value of the structure damage diagnosis factor in the to-be-damaged diagnosis state exceeds the threshold value or not. The method can diagnose damage of all stay cable structures of the bridge at the same time, and is suitable for solving the problem of actual large-span bridge stay cable structure damage diagnosis.