Abstract: Nowadays the inner ET testing is wildly used for in-service inspection of titanium tube of condenser in nuclear power plant. In this paper we analyze the causes of DNT, and consider it is attributed to corrosion inside support plate borehole. We also tested the sample tube with dents by ET tester, we recorded the amplitude of the signal and observed the cracking of the depression, and based on that, we give some suggestions on the treatment standard of the DNT tube in the actual test.