Objective To develop a new method for on-line monitoring of the concentration of nickel ions in nickel plating, and to expand its application to chromeplate, cadmium plating, copper plating and so on. Methods The system was composed by VC++ Language programming, Modbus communication technology and PLC Siemens technology. The amount of nickel plating bath was controlled and revised by meter pump and flowmeter for nickel ion monitoring. The bath solution was automatically monitored by electrochemical testing method and then the results were collected by VC++ Language programming. Thus, the composition of nickel plating bath can be automatically monitored by this excellent and reliable method. Results The relationship between the nickel ions and the polarization current value at -1. 0 V(vs. SCE)was fitted as I=0. 002 01+6. 90×10-5ρ by the system of on-line monitoring of nickel ion concentration. For 120 g / L nickel plating solution, the concentration of the nickel ions was detected as 115. 5 g / L and 113. 7 g / L and the measuring error was 3. 78% and 5. 25% by this on-line monitoring and the EDTA titrimetric analysis, respectively. Conclusion With its high reliability, the system can not only be used to monitor the status of nickel plating bath, but also for the cadmium plating bath and the chromium plating bath. Most importantly, it can be used in harsh environment.