Abstract: During the turbine startup process of one Nuclear Power Plant,the bypass valves vibrated severely,even to break all of the connecting bolts between the valve bodies and the oil-driven actuator.In this paper,a careful analysis of the unit load during severe vibration and the motion curve of the oil-driven actuator,together with the condition of the joint surfaces of the valve bodies and the spools after the split of the valves have been made.Moreover,the authors analyze the structures of the valves and the relative positions between spools and sleeves under different load conditions,and give the origin for the break of the connecting bolts,i.e.,the valve spool of 300kg weight moved back and forth in the pre-inlet travel length and hit the spool,and after 23 min.,the bolts reached the abruption limit and caused four connecting bolts to be broken.In this paper,the recommended modifications are provided.