Abstract: A series of tests are conducted in laboratory and field to investigate the behavior of compacted clayey railroad subgrade subjected to repeated loading of to-and-fro trains. Using the test results, the stress-strain behavior of the subgrade was quantitatively studied, and the causes of diseases in subgrade were analyzed. Results show that the accumulated plastic deformation in fill- subgrade produced by the repeated action of trains wheel-set load is closely related to the saturation of subgrade soil. Specifically, the dynamic strength of subgrade soil decreases sharply with the saturation.