Molecular dynamics (MD) simulations on supersaturated KCl solutions are conducted to understand the homogeneous nucleation mechanism of ions in aqueous solution. According to the MD simulation results, dissolved K⁺ and Cl⁻ ions tend to aggregate in solution. When the aggregates reach a critical size, KCl grains may crystallize inside the aggregates. The free energy surface (FES) during the nucleation process, calculated by the METAD method, demonstrates that the process of ion aggregation does not require the overcoming of a potential barrier, which is different from the two-step nucleation theory. Based on the structural analysis, the formation of aggregates may be closely related to hydrophobic effects. Therefore, the revised classical nucleation theory (rev-CNT) is proposed to understand the KCl homogeneous nucleation mechanism in aqueous solution.