The invention discloses a generator hydrogen cooler on-line leak detection method. The method comprises the following steps that S1, a leaked hydrogen cooler is isolated; S2, residual water in the isolated hydrogen cooler is drained, and a connection flange of water inlet and outlet of the hydrogen cooler is disassembled; S3, water chamber cover plates on the two sides of the hydrogen cooler are opened, and a water pipe of the hydrogen cooler is exposed; S4, the two ends of the cooling water pipe of the hydrogen cooler are blocked with sponge rubber balls soaked in soapy water, the water pipewith bubbles is record, red copper plugs are used for blocking the two ends of the leaked water pipe, and then the soapy water is brushed at the sponge rubber balls to detect whether or not bubbles exist until no bubbles exist at the sponge rubber balls; S5, after a period of time, the sponge rubber ball on one side of the cooling water pipe is removed, the hydrogen concentration in the cooling water pipe is detected through a hydrogen leak detector, the water pipe with alarm is recorded, and red copper plugs are used for blocking the two ends of the leaked pipe. The method is also applicableto leak detection on the hydrogen cooler when a generator stops.