The reasons of root discontinuity on the tube-to-tube sheet weld test coupon of the steam generator in a domestic improved second generation nuclear power unit are analyzed. The results show that the tube is not being thoroughly cleaned,so something on the tube wall influence the tacking expanding and lead to a excessive gap between the root tube and hole,which is the ultimate reason of the root discontinuity. So the weld test coupon could not represent the production welds. According to the analysis result,corresponding precautionary measures are proposed,which has some reference for the subsequent welding of the tube-to-tube sheet welds.