Abstract: The feedback from foreign countries shows that the performance of siphon-breaker pipe in some nuclear power plants is insufficient and potential safety risk. In the present study, numerical simulation was carried out with RELAP5 code to verify the performance of siphon-breaker pipe of PTR system for a typical nuclear power plant. The results show that under the present design siphon-breaker pipe can’t break the siphon flow in time, as the results, the coolant in the spent fuel storage pool will be continuously drained out and can yield the exposure of the coolant pipe inlet, and the cooling function of the used fuel storage pool is lost, so that huge safety risk of nuclear power plant is created. The coolant level decreased in spent fuel pool is affected by the height difference between rupture location and free level in pool, flow resistance and pipe configuration of PTR system. The flow resistance of PTR pipe can alleviate the consequence of siphon flow by pipe rupture.