For detecting requirements of Converter Valve leak in HVDC converter station , propose a detection method based on light fiber loop. The detector includes two parts: water level gauge and Detection module, which are conected by fiber. Detection module emits light pulse and receives it. The fiber loop would be cut by water level gauge when valve leak, Detection module creats alarm information as soon as it can’t received light pulse. The function design is effective and reliable, it’s satified requirement of project. The decive has been used for detecting Converter Valve leak in several converter stations including HA-ZHENG HVDC .