In order to prepare the building blocks with middle refractive index for fabricating colloidal crystals, a new strategy of cationic ligand protection (CLP) was developed for one-pot preparation of narrow-dispersed CAS spheres with tunable sizes (94-303 nm) and good aqueous dispersibility. The key of CLP strategy is controlling the aggregation behavior of small US nanocrystals (NCs) via automatic control of the ligand modification condition at different reaction stages, which is realized by selecting gradual decomposition of thioacetamide (TAA) as the anionic precursor and cetyltrimethylammonium bromide (CTAB) cations as the ligands for anions. Accordingly, at the initial stage of the reaction when TAA decomposes incompletely, the surface atoms of the US NCs are mainly Cd cations, leaving less anionic sites for the coordination of ligands. The poor ligand modification condition of US NCs still leads to their aggregation. Along with the decomposition of TAA, S anions and cationic ligands gradually dominate the surface of US aggregates, providing the aggregates sufficient protection, and thus, they are stably dispersed in water. The current CLP strategy simultaneously involves the processes of NC formation and NC assembly and therefore promotes the one-pot preparation of NC aggregates. The as-prepared US spheres possess uniform size, good water-dispersibility, and relative higher refractive index and thus can be applied for fabricating colloidal crystals.