The invention relates to a circulating water chlorine adding system and method. The system comprises seawater pressurizing pumps, dosing pumps, dosing mother tubers, circulating water front tanks and circulating water shields which are identical in number and further comprises a PLC (programmable logic controller) unit, a first input end of each dosing mother tube is connected with an output pipeline of the corresponding seawater pressurizing pump while a second input end of the same is connected with an output pipeline of the corresponding dosing pump, an output end of each dosing mother tube is connected with input pipelines of the corresponding circulating water front tank and the circulating water shield, and the PLC unit is in communication connection with the seawater pressurizing pumps and the dosing pumps and remotely controls the same to start to add chlorine or stop. A control system is utilized to replace manual operation, and an effective chlorine adding metering monitoring means is set, so that accuracy and controllability of circulating water chlorine adding metering are realized, labor intensity and misoperation are reduced effectively, and safety and reliability are improved.