Abstract: A brief description of the principle, detection equipment and technical indexes of neutron method was used to detect the inner cavity defect of concrete under steel palte lining. The specific introduction of site testing conditions and results of 7 runner chambers in Shuikou hydro-power plant were described in this article. The results showed that each runner chamber had several pieces or isolated inner cavity defects, which depth were less than 3 mm. Except 1# runner chamber, all of the rest had isolated inner cavity defects which depth were within from 3 mm to 10 mm. Affected by falling of steel plate, the depth of inner cavity defects in 3# runner chamber were larger than 10 mm. A remeasurement for 3# runner chamber after grouting was carried out, and summarized guidance for the retested results. In addition, the reasonable analysis was did in the aspect of grouting, the water content of the background and cavitation for the high thermal neutron count of 3#, 4# and 6# runner chamber. The successful application of neutron method for detecting inner cavity defect of concrete under steel plate lining of the runner chamber provides a reliable means of detection and scientific basis to the construction and operation safety of permanent components embedded in concrete in water conservancy projects.