Abstract: The aluminium in water can be reacted with Beryllium ReagentⅢ forming a stable and red complex,by heating in buffer solution of acetic acid with pH 6.1 at 85℃ for 5 min with water bath. The complex possesses complex ratio 1:1,condition stability constant lgβ=10.38 and molar absorption coefficient ε=1.08×10⁵ L/(mol·cm). In accordance above,a new way of determining aluminium element with content 0.01～0.2 mg/L in water have been established.The method could be applied to aluminium determining of water long contacted with protecting materials in transporting and distributing engineering, drinking water as well as other water resources. A larger number of experiments proved that good determining results can be obtained by this method.