The invention discloses a leak-detecting method for a cooling system of a blast furnace, which comprises the following steps: firstly, confirming a leaking cooling pipe, connecting a transparent pipe with a vent valve of a section of cooling pipe at the middle part, and closing a communicating valve at the lower part of the section of cooling pipe at the middle part; then closing a return valve of the leaking cooling pipe, and opening the vent valve of the section of cooling pipe at the top of the leaking cooling pipe; raising the pipe end of the transparent pipe to the position of the vent valve of the section of cooling pipe at the top of the leaking cooling pipe, opening the vent valve connected with the transparent pipe to lead water in the leaking cooling pipe to freely decline and keep synchronous with water level indicated by the transparent pipe, and further confirming a water leaking position; and if the water level declines all the time, and till the joint of the transparent pipe and the vent valve still shows no signs of stopping, the connecting position of the transparent pipe can be moved downwards, and the method is repeated for continued observation until the water level stops declining, thus being capable of confirming the water leaking position. The method is convenient for operation and easy to master.