The effect of initial concentration and pH value of the uranyl nitrate solution on the uranyl ion (UO2+2) hydrolysis reaction was studied by spectrophotometry. The maximum absorption wavelength of the obtained complexes in the reaction of UO2+2 ions with arsenazo (Ⅲ) at different pH values was also investigated. The results show that the absorption wavelength is 652 nm in the range of pH 3.0~7.0. It is easier for the uranyl nitrate solution to hydrolysis at higher pH value than the lower in the range of pH 4.0~6.0 at the same UO2+2 concentration. The uranyl solution at high concentration is prior to occurring hydrolysis reaction in the same pH value. Lowest pH values of various uranyl solutions were obtained for hydrolysis reaction. The results indicate that the concentration of uranyl solution should not exceed 1.000×10-3 mol/L at pH 5.0.