A DDES method, which is a hybrid RANS-LES method, is proposed for numerical simulation of Mars Exploration Rover heat shield separation. The heat shield and back shell drag characteristic and aerodynamic interference effect between them are analyzed at different axial separation distances. The computational results are compared to the experimental results. And then security analysis of heat shield separation is discussed. The results indicate that the aerodynamic interference effect of heat shield and back shell is most serious inducing a suction force while the axial distance is less than two heat shield diameters. And there will be a recontact risk after heat shield separating while the axial distance is less than five heat shield diameters. To ensure successful heat shield separation, the axial distance must be larger than five heat shield diameters.