Ca²⁺ ion is a kind of common impurity in the growth process of KDP crystal. Effect of Ca²⁺ ions on the growth habit and optical properties of KDP crystals were investigated. KDP crystals were grown from a solution containing certain quantity of calcium chloride by the traditional temperature-lowering method and by the “point seed” technique, respectively. The results show that there is no obvious effect on the stability of KDP growth solution and the process of KDP growth when the Ca²⁺ concentration in the solution is very small. When the Ca²⁺ concentration is high in the solution, the stability of KDP growth solution is lowered, and inclusions in the crystal often occur on the prismatic faces of KDP crystals grown by a “point seed” technique. The ultraviolet transmittance is descended and the density of scattering particles in the crystal is increased with the Ca²⁺ concentration in the solution.