The invention discloses a method for detecting ballastless track structure interlayer damage. Disengagement and open joint damage on a single part and multiple parts of an interlayer of a ballastless track can be identified according to a track slab Gaussian curvature index. The method is characterized by comprising the following steps: solving a displacement mode value of a track slab in any order mode through modal analysis under given boundary conditions; and further solving the Gaussian curvature of the track slab according to a displacement mode so as to reflect transverse and longitudinal curvature modes of the track slab and determine the presence of damage. Based on a modal analysis theory, whether damage exists or not and the damage position can be identified through former third-order Gaussian curvature indexes of the track slab, and particularly, the first-order Gaussian curvature index is the most obvious. The method is a nondestructive testing method, the testing is simple and convenient to operate and is not influenced by steel bars in the track slab, the detection result is intuitive and clear, whether damage exists or not can be identified, and the position of damage can be accurately positioned.