This paper addresses the challenge of accurately describing the boundary of deep cavern-type reservoirs. A method is developed to extract diffraction information from the cavern and its boundaries from full wavefield seismic data using PCA wavefield separation technology. The paper describes a method for extracting diffraction information based on post-stack seismic data, and demonstrates the validity of this method in identifying cavern’s boundaries via forward modeling. Subsequently, the method is applied to actual seismic data to extract diffraction information from deep caverns. By separating wavefield information at different scales, the extracted diffraction information can effectively identify the characteristics of cavernous reservoirs and their boundaries. It is verified by examples that the diffraction wave information separation method can provide a more accurate description of the distribution of deep cavern-type reservoirs, which can provide a basis for predicting this type of reservoir.