For the vibrationdamped composite plate samples, the sample size standards are different in the damping test, and the sample size lacks the theoretical basis in the determination, resulting in that the size of the test sample cannot be determined. The damped composite plate was modally analyzed by the finite element method. The cantilever beam method was used to measure the damping. The finite element simulation results showed that, with a certain width and thickness, the resonance frequency of the 1st to 6th order was decreasing with the increase of the length of the sample. Under a certain length and thickness, the first four order resonance frequencies slightly increased with the increase of the sample width. At a certain length and thickness, as the sample thickness increased, the resonance frequency of the 1st to 3rd order remained basically stable. The cantilever beam method was used to measure the damping. The measured results showed that the sample with a free end of 216 mm in length and a width of 12.7 mm was characterized by the loss factor at the 2nd and 3rd order resonance frequencies.