Quantitative changes of Cd2+ in drinking water system could reflect directly or indirectly the risk of safety in the drinking water. Therefore, possibility for predicting rapid increases of Cd2+ in the drinking water by P-matrix model was investigated. At the buffer solution (pH 9.0) of NH4Cl-NH3•H2O, the Zn2+, Cu2+, Cd2+ were colored with 2-((Bromo-2-pyridyl)-azo)-5-diethyl-aminophenol, in the presence of Triton X-100. Under the absorbent spectrum of 400~700 nm wavelength, the recoveries of Cu2+ was close to 100%, and that of Cd2+ was 95.8%─104.0% when Cu2+ and Zn2+ were in low concentration,showing that the P-matrix model construction could predict successfully for the quantitative change of Cd2+ in the drinking water system.