The large deviation between test results and real conditions caused by the coupling of test fixture is unavoidable. The research on free interface modal synthesis method can gain the modal test boundary transformation of one missile wing. For this purpose, the low order normal mode spectrum matrix and mode shapes was gained by free modal test, the extra high orders modes can be obtained through FEM analysis, while the boundary constrains was equivalent as a kind of sub-structure, and the modal synthesis was used to extract the constrained modal parameters. The experiment results are consistent with numeric calculation results very well, which indicated that this extracting method is feasible.