The invention relates to a system and method for controlling the concentration ratio of an open type circulating cooling water system. The system for controlling the concentration ratio of the open type circulating cooling water system includes a control system and a cooling tower; a cooling tower water tank with a cooling tower water tank liquidometer is arranged below the cooling tower; the bottom of the cooling tower water tank is provided with a pollution discharge pipeline; the pollution discharge pipeline is provided with a circulating water conductivity meter, a circulating water temperature gauge, a pollution discharge electric control valve and a pollution discharge flowmeter; a water entering pipeline used for supplementing the cooling tower water tank with water is arranged above the cooling tower water tank; and the water entering pipeline is provided with a water supplementation flowmeter, a water supplementation conductivity meter, a water supplementation electric control valve and a water supplementation temperature gauge, wherein the water supplementation electric control valve is connected with the cooling tower water tank liquidometer. With the system and method of the invention adopted, automatic continuous and stable water supplementation and pollution discharge of the open type circulating cooling water system can be realized; automatic continuous on-line monitoring on the concentration ratio of circulating water can be realized; and concentration ratio fluctuation which is caused by intermittent water supplementation and pollution discharge of the circulating cooling water system can be avoided, so that the quality of the circulating water can be stabilized.