The surface of the embankment slope of earth-rock dams is typically under low stress conditions. Studying the dynamic characteristics of coarse-grained materials under low stress and proposing a method for identifying the loosened zone of earth-rock dams are of significant theoretical and practical importance for the design and safe operation of these dams. This paper combines physical experiments and numerical simulations to explore the dynamic characteristics of earth-rock dams under seismic loading and proposes a new method for identifying the loosened zone and its application. By analyzing the relationship curve between vertical displacement and shear strain，this paper provides criteria for distinguishing the compaction and dilation behaviors of coarse-grained materials under low stress and further presents a method for identifying the loosened zone. This method is based on the relationship between the critical shear strain and vertical stress obtained through experimental fitting，which is used to determine the average stress and dynamic shear strain of each unit，thereby achieving accurate identification of the loosened zone in earth-rock dams. The research results can provide reference for the identification of loose areas in earth-rock dams.