The invention relates to the technical field of a nuclear power plant, in particular to an online pressure seal overhauling method of a primary loop check valve of the nuclear power plant. According to the method, isolated overhauling of the valve is innovatively completed under the condition that cooling water in a primary loop system is prevented from being lower than lower low water level, the cooling water in the primary loop system is always higher than the lower low water level in an overhauling process, so that water filling and gas exhausting steps for dynamic gas exhausting or vacuum pumping at a startup stage of a unit are avoided, overhauling of the primary loop valve is really realized without the lower low water level, the manpower and material cost for overhauling is saved, the overhauling duration is greatly shortened, and the overhauling cost is reduced.