Abstract: This paper mainly focuses on resolving the numerical weather prediction problem in the steep orographic region of China. The stepped terrain vertical coordinate (known as Eta) is introduced into the dynamic core of the weather research and forecasting (WRF) model to improve numerical forecasting in the complex orographic region. We designed the atmospheric column mass transformation scheme to give the Eta WRF core the same model equation system formulation as the Sigma WRF core, which can facilitate discretization and programming. The design of the stepped terrain follows the computational scheme of the advanced regional Eta-coordinate model (AREM), including the introduction of the reference atmosphere, the adoption of mesh description indicators, and the representation of stepped mountains. We performed 2D idealized mountain wave simulations and 3D real-case experiments to test the correctness and robustness of the new dynamic frame. Inherent isolation of the stepped topography can result in airflow separation, which accounts for inadequate simulation of the mountain wave with a coarse vertical layering using the new Eta WRF core. In this case, the mountain wave simulation will improve using the new dynamic frame either because of the fine vertical resolution or the long simulation time, which is considered alleviation on the airflow isolation in stepped mountains.