The accuracy and applicability of wall boundary models in fluid simulations are crucial for accurate calculations of forces exerted between the flowing fluid and the wall boundary, determining the reliability of results throughout the entire computational domain, including the inner region of the fluid. In light of the above, this study proposes an extended WPP (Wall Potential Particle) model as an accurate wall boundary model targeting the projection-based SPH (Smoothed Particle Hydrodynamics) methods [1,2], i.e., the ISPH (Incompressible SPH) method that enables more accurate analysis of incompressible fluids. The existing WPP model [3] is only applicable to MPS (Moving Particle Semi-implicit) type of kernels, which take an infinite value at the centre, and thus cannot be applied to SPH methods, which employ kernels taking a finite value at the centre. The proposed model overcomes this difficulty and can be implemented into the SPH method regardless of the kernel shape. The conventional wall boundary condition applied to the SPH method uses fixed particles [2,4], so it suffers from the problem that unevenness occurs at the wall surface, resulting non-physical friction on fluid particles. Furthermore, it leads to the fluctuations of the particle number density at the scale of the particle diameter, inevitably introducing noise to the predicted velocity and pressure near the wall. Benchmark tests demonstrated that the proposed model resolves such issues, enabling more accurate calculations. The detailed results will be presented at WCCM 2026.