Abstract: The item safety classification of nuclear power plants has to comply with the basic principles presented in relative rules and codes, while there is no uniform conclusion for the method of safety classification, so much the worse for that of system I＆C classification. The paper gives an implemention method for the system I＆C classification, namely, based on the system function classification and system operation analysis, identifying the safety signals of system in accident conditions, and distinguishing the control process of the relative equipments, then the system I＆C classification is determined based on the classification of system function which the signals and control process are performed. The implementation of the method on the I＆C design of essential service water system（SEC） in Hongyanhe nuclear power plant is reviewed.