Internal erosion, caused by seepage flow inside the soil, accelerates soil failure during a natural disaster. Numerical simulation, which enables microscopic observation through hypothetical numerical experiments, can be an effective tool to quantitatively evaluate the relationship between internal erosion and the instability of the ground as a whole. Internal erosion and multiphase flow simulation of fluid and granular materials with a particle size distribution require coupled simulations that can represent the interaction between particles and pore water and the movement of particles. There are two main types of coupled models: "Resolved coupling model," which can calculate detailed flow and fluid forces, and "Unresolved coupling model," which is based on empirical drag and seepage flow models. However, previous studies have indicated that both models should be judged appropriately based on the ratio of particle to fluid spatial resolution. However, applying a coupled resolution model to the vast number of soil particles that make up the ground is impractical from a computational cost perspective, and empirical unresolved coupling model has difficulty in representing localized failures such as internal erosion. Therefore, developing a new coupling model that satisfies both computational accuracy and efficiency is desirable. In this study, we applied ISPH for fluid analysis and DEM for soil particles to develop a fluid-soil coupled simulation model that can directly represent the movement of soil particles during the internal erosion process. A coupled fluid-soil simulator that can directly represent the movement of soil particles during the internal erosion process is constructed. Through numerical experiments using a particle layer with the vertical upward flow, we understand the limitations of the conventional coupled model and propose a new hybrid type of semi-resolved coupled model that combines these two models appropriately.