Abstract: A carbon fiber/polymer smart layer able to sense dynamic strain was developed to perform structural strain modal diagnosis. The sensitivity of such a smart material to dynamic load at low frequency was revealed by the tension tests conducted with different frequencies, and the influencing factors on the error of the dynamic response were analyzed theoretically. The smart layer was continuously laid on a cantilever beam and took place of traditional strain gauges to test its strain modals. The test results indicate that the smart layer is able to exactly present the natural frequencies and the strain modal shapes of the first three orders for the beam. The test was conducted again on the beam with added mass. It is found that the added mass leads to the decrease of the natural frequencies and induces mutations on the smart layer characterized strain modal just in the position where the mass added. The results demonstrate the ability of the smart layer characterized strain modal to identify the change of the structural physical parameters. Good agreement was achieved between the results attained with the smart layer and the strain gauges. In addition, based on the ability of covering in smart layer, the information about the structural strain modal is fully captured. Furthermore, the change of the structural physical parameters is located by the tests with added measured points in the suspected area.