A general construction approach for a class of detached-eddy simulation (DES) methods with the turbulent length-scale equation is presented in this study. It supports the rationality of the construction and provides a theoretical estimation of the model coefficients in DES simulation. By using the construction approach, a differential Reynolds-stress model (RSM), referred to as Speziale-Sarkar-Gatski (SSG)/Launder–Reece–Rodi (LRR)-ω RSM, is built into the improved delayed DES (IDDES) method. After calibration of the model parameters in the IDDES method and basic validation for decaying isotropic turbulence, the RSM-based IDDES approach is then applied to simulate the massively separated flows around the tandem cylinders and the transonic buffet flow over a hammerhead launch vehicle. The simulations are validated by the available experimental data, and the performance is evaluated by means of instantaneous, statistical, spectral analysis of the numerical data. It is found that the RSM-based IDDES method shows better performance comparing with the k-ω shear-stress transport (SST)-based IDDES method, especially for predicting the development of massively separated flows behind the bluff body.