Over the past decades, geohazards such as landslides, debris flows, floods, and rock avalanches have grown in frequency and intensity, often involving complex multiphase and multi-material interactions. In many cases, one event triggers another, amplifying the overall impact. For instance, an intense earthquake may induce a landslide that dams a river, ultimately causing severe downstream flooding. This highlights the urgent need for more advanced forecasting methods, robust hazard assessment tools, and effective mitigation strategies. However, capturing the intricate processes of these events remains a challenging issue. Discontinuous Deformation Analysis (DDA), as a family member of the Discrete Element Method (DEM), has proven powerful in geohazard research due to its ability to model polyhedral blocks and structural elements.[1] Recently, DDA has been coupled with Smoothed Particle Hydrodynamics (SPH) to simulate fluid–solid interactions, demonstrating promising potential for complex geohazards. [2-3] This study presents an improved DDA–SPH method for simulating multiphase geohazards. Improvements to contact detection and interaction calculations have been implemented to boost both efficiency and accuracy. In this coupling method, discontinuous materials (such as rock blocks and rigid barriers) and deformable structures are modelled using DDA, while flow-like materials (such as fluids and soils) are represented by SPH. The faces or edges of DDA blocks serve as moving boundaries for the SPH particles. Multiple constitutive models, including Newtonian and non-Newtonian fluids, soil, and brittle solids, have also been integrated into the SPH framework, and advanced geological considerations have been incorporated into the DDA framework, such as velocity-dependent friction laws in landslides. Simulations using this improved DDA–SPH method demonstrate robust performance in modeling dynamic multiphase geohazards, including landslide-induced tsunamis, landslide dams, and debris flow impacts on structures. It provides a powerful tool for deeper mechanism investigation, more reliable disaster prediction, and the design of more effective mitigation and prevention measures.