A Kalman filter spectrophotometry based on interval selection of correlation coefficient threshold method is proposed to simultaneously detect trace ions of copper, cobalt, and nickel in industrial wastewater without any separation steps. Firstly, 40 groups of mixed standard solutions of copper, cobalt, and nickel are prepared, and the absorbance coefficient matrix is solved by multiple linear regression method. Then, a correlation coefficient threshold method is proposed to select the wavelength region with high sensitivity for filtering. Finally, the filter end point is determined by the filter variance to obtain the estimated mass concentrations of copper, cobalt, and nickel. The linear detection ranges of mass concentrations of copper, cobalt, and nickel are 0.5-5.0, 0.2-2.0, and 0.3-3.0 mg/L, respectively. The average relative errors of copper, cobalt, and nickel are 2.862%, 2.464%, and 3.781%, respectively, which are all less than 5%. The predicted root mean square errors of copper, cobalt, and nickel are 0.1124, 0.0279, and 0.0663, respectively. The results show that the proposed method is simple and rapid, can filter while scanning, and is easy to analyze on-line.