Abstract: This paper describes a case testing by using neutron method for detecting inner cavitydefect in pour concrete under steel plate lining of four hydro-power generation draft tubesin Gusngxi Guigang navigation project. The test results can provide very useful information for guiding grout construction and for Inspecting grout quality. This new neutronmethod is a non-destructive,qulck reliable and safe testing technique for concrete construction quality inspection.