In this paper, a DDES analysis on the aerodynamic forces and moments of a generic missile inside a rectangular cavity is presented. First, the implementation of the DDES method into a CFD code is validated by comparing the computational results with the experimental results for unsteady turbulent flows around a cylinder and a back facing step. Second, the unsteady pressure fluctuations inside WICS cavity are computed and compared to the experimental results, which shows a good agreement. The unsteady flow field inside the cavity with the missile is analyzed, and the computational results of the unsteady pressure fluctuation and the force and moment are presented.