To overcome the limitation of random method that is not suitable for the case of small sample, the objective unascertained information is fully used, and the dynamic characteristics analysis model of space beam-plate composite structures is built, in which structural elastic module and mass density are all considered as unascertained variables. And a structure analysis method based on the unascertained factor method is given. Firstly, the structural element stiffness and mass matrices are built based on the mathematics expression of unascertained factor. Then by using the arithmetic operation rules of unascertained rational numbers, the computational expressions of dynamic eigenvalue for the beam-plate structures are derived from the Rayleigh's quotient. It is concluded that the confidence level of structural responses are in direct proportion to the confidence level of each parameter. Comparing with the mature random method, the proposed method is able to obtain reliability result of safer and higher faith degree in the case of inadequate data or insufficient information; moreover, it is simple and easy to apply. At last, an example is given, in which the possible values and confidence levels of natural frequencies for the unascertained beam-plate composite structures are obtained. The rationality and validity of presented method are demonstrated.