In this study, based on Smoothed Particle Hydrodynamics (SPH) and Discrete Element Method (DEM), a multiphase fluid-solid coupling algorithm of SPH-DEM is provided, and a high performance coupling module is developed based on two open-source software platforms, namely: DualSphysics for SPH and Blaze-DEM for DEM. With a comparative analysis of three typical tests, it is systematically proved that the developed coupling module can feasibly and precisely tackle the complex coupling between solid particles and fluid. Moreover, based on the test results of single particle sedimentation and water-entry of spinning spheres, three water-entry stages of the spheres are characterized: contact and shockwave generation, cavity formation and water splashing, and cavity closure and collapse. As a complex problem, the submerged granular collapse is simulated, and according to the numerical results the process of granular collapse can be divided into four stages: beginning, collapsing, decelerated sliding and stable sedimentation. The granular aggregation can be partitioned into the zones of flowing, sliding, shearing, and relatively standstill particles by the motion characteristics of different parts of particles on different stages. In addition, the macro- and meso-characteristics of the process of granular collapse are also discussed. According to the research results of this study, the developed SPH-DEM module can better simulate the complex coupling process between fluid and granular materials. Furthermore, with more information obtainable, numerical simulation is important to get insight into this process. Display omitted •The SPH and DEM coupling routines are presented.•A 3D SPH-DEM coupling engine is developed based on two open source codes.•Three typical tests have been used to validate the coupling engine of SPH-DEM.•The water-entry of a single sphere can be divided into three water-entry stages.•The submerged granular collapse is simulated and the process of the collapse are discussed at macro- and meso-scale.