Impurities significantly constrain the production of high-purity magnesium sulfate crystals, essential for advanced magnesium-based materials. Sodium–magnesium co-precipitation affects the crystals’ purity and surface smoothness, with NaCl embedding into crystal surfaces during cooling crystallization in the MgSO4-NaCl-H2O system. Trace impurities, however, inhibit NaCl adhesion, alter SO42− coordination structures, and enhance the purity and morphology of MgSO4·6H2O crystals. Adding 300 mmol/L K2SO4 reduces sodium content in crystals from 0.57% to 0.03% and surface roughness from 2.76 nm to 0.415 nm. The binding energies of Na+ on MgSO4·6H2O crystal planes are lower than those of impurity ions, which compete for active growth sites and prevent Na+ nucleation. This finding challenges the assumption that higher-purity solutions yield higher-purity crystals.