The reagent p-Br-arsenazo, can react with calcium canon (Ca²⁺) in strong basic medium and form:stable green complex with a maximum light absorption at 635nm, The molar extinction coefficient is 9.0×10³, The curve of standard concentration of Ca²⁺ is a straight line in the range of 5-40μg Ca²⁺/25 ml, The interference of Fe³⁺ and Cu²⁺ can be eliminated by adding F^-(or PO₄^3-)and ascorbic acid respectively, while Ag⁺, Co²⁺, Sn⁴⁺, Bi³⁺, Cr³⁺ can be screened by adding trietha-nolamine, The relative errors for determination of Ca²⁺ in well water and extracting of soil samples range from -5.33% to 7.96%.