Abstract: The gaps among piping and its supports are usually neglected when analysing the seismic stress for the secondary piping in nuclear power plant (NPP), but the gaps exist undoubtedly except the anchors. The results comparison was made under the seismic load with and without gaps for the piping PTR335-3/4″ in Daya Bay Nuclear Power Station. The influence of the support gaps to the seismic stress of piping was researched. The results indicate that the stress distribution, the location and the value of the maximum piping stress are quite different, so the effect caused by the gaps should be fully considered in the seismic analysis for the secondary piping in NPP.