This paper proposes a method to estimate the instability of the slope surface layer by elastic wave attenuation which changes with soil moisture and deformation. The objective of this study is to investigate the effect of soil moisture and deformation on wave attenuation of soil. Two experiments were conducted, the first one was a laboratory experiment using a multi-layer shear model, and the second one was an on-site monitoring wave attenuation with soil moisture. In laboratory experiments, wave attenuation at various soil moisture and deformation were measured. The relationships between wave attenuation and soil moisture were found to have hysteresis, that is, difference paths were observed in rain and drain process. The attenuation of wave energy increased about 20%~40% when the VWC grew up near saturation. With increasing the displacement, the wave attenuation also increased, and the wave energy ratio dropped by 40% during a 3 mm of displacement. The results of on-site monitoring showed that wave attenuation decreased with the increasing soil moisture in the rain events and increased during the drain stages. Since most of the rain-induced landslides start failure in nearly saturated conditions, and the elastic wave attenuation in soil can indicate the status of saturation, monitoring elastic wave in the surface layer of slope can detect its instabilities.