Multi-directional propagating nonlinear internal waves (NLIWs) create complex spatial patterns, often making it difficult to quantitatively estimate the reflection and dissipation processes of NLIW energy over a slope. To identify the onshore- and offshore-going wave signals in a regional model, we apply a directional Fourier filter (DFF) method to clarify wave dynamics over varying slopes. First, a series of two-dimensional analytical solutions are utilized to demonstrate the well performance of DFF method, regardless of the bidirectional solitons or NLIW packets. Next, two-dimensional numerical experiments indicate that the rate of reflection (dissipation) of energy for the shoaling NLIWs is much lower (higher) than that for the shoaling internal tides over a slope, regardless of varying slope criticality and height. Lastly, we apply the DFF method in a three-dimensional non-hydrostatic regional model (MITgcm) to directionally decompose the onshore- and offshore-going internal waves (IWs) on the Australian North West Shelf. The model results show that mode-1 incoming internal tides gradually steepen into NLIWs during the shoaling processes over the slope, and then the reflecting IWs are propagating offshore in a main form of linear IW beams. In addition, the reflectivity of IWs around the Imperieuse Reef is 60% and the offshore-reflecting IWs quickly dissipates accompanied with an e-folding length scale of ~22 km.