Accurately modeling dense granular flows is essential for two-phase sediment transport simulations. This study introduces an improved μ(I) rheology model incorporating a novel L1-regularization technique. First, the numerical limitations of the standard μ(I)-rheology are identified, followed by the derivation of a new regularization strategy. Second, the technique is integrated into the Material Point Method (MPM) framework to solve dense granular flows problem with the large deformation characteristic. Results demonstrate that the regularization significantly improves system conditioning during simulations, achieving strong agreement with experimental data and alternative numerical methods.