Abstract: P-Acelyl-arsenazo reacts with calcium cation at pH=12.9, forming a dark-blue complex with maximum absorption at 640 nm. The molar extinction coefficient of the complex is 1.1×10⁴. The relationship between the absorption and the concentration of the solution obeys the Beer's law, provided that Ca²⁺ has the concentrations within 5-50μg/25ml.The interference of Fe³⁺ and Cu²⁺ can be eliminated by adding F^- and triethanolamine,respectively. The recovery of Ca²⁺ added into the solutions of digested grains varys in the range of 95-110%.