The invention relates to an on-line monitoring system and a monitoring method for cold water in a convertor station. The system comprises a sampling system, a constant temperature system, a data acquisition and analysis system and a transmission system, wherein the sampling system is used for acquiring circulating water from a main circulating pipeline of the cold water in the convertor station, wherein the circulating water is used as sampling water; the constant temperature system is used for keeping the temperature of sensors of a measurement system within constant temperature; the data acquisition and analysis system is used for arranging all the sensors in the sampling water in the constant temperature system, transmitting information acquired by the sensors to a data analysis system through transmission lines and performing operational analysis; and the transmission system is used for transmitting results of the data acquisition and analysis system to a remote client through a network. The on-line monitoring system and the monitoring method have the following advantages that: a core test module can be replaced and the quantity of the sensors can be increased according to specific test content; and manual intervention is not required, various indexes of the cold water in the convertor station can be regularly monitored in real time, and the basis of establishing the optimal operating conditions of a circulation system of the cold water in the convertor station is provided.