Surface nuclear magnetic resonance(SNMR) technique is a non-invasive detection technology, which, through the recognition of the distribution of hydrogen atom signals in water molecules, infers the distribution of water molecules in the strata, and then determines whether there exists groundwater or not and provides some information concerning the water table, the water-rich degree and the porosity. In this paper, taking a soil dam of a reservoir in Anhui Province as an example, the authors used the SNMR to investigate the dam leakage, and obtained such physical parameters as initial amplitude (E₀), longitudinal relaxation time (T₁), and transverse relaxation time (T₂^*). Through analyzing the correspondence between the physical parameters and the geological structure, the authors determined the existence of two leakage points and, through field investigation and engineering geological drilling, verified the geophysical survey results, thus providing the geological basis for the management of dangerous reservoir dam.