Abstract: The characterization test method of nonlinear internal resonance for a fiber-reinforced composite thin plate (FRCP) based on integer-multiple coordination technique was proposed. Firstly, in order to make the first two natural frequencies of such a composite plate have a ratio of 1:2 or 1:3, the explicit expressions of internal resonance frequencies were derived, and the dimension parameters conforming to the formation condition of internal resonance phenomenon were also pre-determined. Then, the characterization test principle of internal resonance of FRCP was illustrated, so that a reasonable and standard test procedure can be summarized. Finally, the TC300 carbon fiber/resin composite plate was taken as a research object, and its 1:2 internal resonance phenomenon was studied. It has been found that when the composite plate was excited by the second natural frequency, the nonlinear vibration energy transfer behavior can be discovered under 1:2 internal resonance state. However, with the increase of exciting level, the resonance amplitude at the lower natural frequency would decrease, compared with the one of higher natural frequency, i.e., the internal resonance phenomenon of FRCP was becoming weak, and its vibration motion gradually changed into a singular period.