Because of the existence of the Faraday rotator mirror，the sensing signal in the optical fiber hydrophone based on Michelson interferometric optical path is disturbed by the strong electromagnetic field. The improved sensing optical path structure separates the Faraday rotator mirror from the optical fiber hydrophone, and the sensing signal can be resistant to the strong electromagnetic field. A new type of optical fiber hydrophone is made. The optical fiber hydrophone which is installed in a large transformer can detect noise inside the transformer under different load currents successfully.