Abstract: For the representative failure happened to the reactor coolant temperature detector in Qinshan Phase Ⅱ Nuclear Power Plant, we discovered the cause of failure and tried to solve this problem by analyzing the structure of material, the situations in other nuclear power plants, the vibration in laboratory and in field. The analysis shows that the reason is the collision happened between the protective well and the temperature detector. The temperature detector will be broken or ruptured. Adding the ring outside of the temperature detector is proved to work well.