The impact-echo method, which is to evaluate the integrity of concrete and masonry structures nondestructively, is an excellent method in practical applications, and provides a high quality of structural integrity assessment. However, in the case of multi-layered systems in which each layer has different stiffness, the impact-echo method may lack reliability in thickness evaluation, which demands improvement of the impact-echo method. This study was first dedicated to the understanding of stress-wave propagation in the impact-echo test, and secondly, the reliability of the impact-echo method was investigated through the numerical simulation of the impact-echo test. The investigation included the research on influencing factors such as stiffness contrast between layers and receiver location. Finally, the research in this paper led to the development of the phase-difference response (PDR) method, based on the frequency response between two receivers deployed in a line with an impact source.