A CFD-DEM particle dissolution model was established in this study to simulate the dissolution process of particles in a stirred tank. The model uses an improved porous sphere model to calculate the volume fraction of the particles, and optimizes the momentum source term accordingly to make the numerical calculation more stable. Firstly, the model's accuracy in simulating particle size and solute concentration was validated against experimental results, the experimental errors were 6.13% and 12.5%, respectively. Secondly, the study delved into the mechanism of mass transfer dissolution from three dimensions: solute concentration distribution, particles spatial distribution, and energy dissipation rate of particles. It was found that the larger the concentration gradient, the better the dissolution of particles. The study shows that the larger the surface concentration gradient of particles in the region with lower content concentration in the stirred tank, the better the dissolution effect of particles. With the process of particle dissolution, the mixing degree of particles and fluid is getting higher and higher. However, due to the slow shrinkage of particles, the coupling force between fluid and particles is gradually weakened, and smaller particles are more easily suspended. The energy dissipation rate will affect the mixing behavior of particles and fluids in local regions. The higher the local energy dissipation rate is, the faster the mass transfer rate of particles surface. Lastly, the dissolution behavior of particles at different rotation speeds was investigated. It is found that higher stirring speed can significantly improve the dissolution rate of particles in the particle suspension and dispersion stage, but the difference of different rotational speeds on the dissolution rate becomes smaller in the particle circulation flow stage. By analyzing the input power and particle dissolution rate of the mixing tank comprehensively, selecting the appropriate mixing speed can effectively save energy consumption.