Spectral similarity comparison through tandem mass spectrometry (MS2) is a powerful approach to annotate known and unknown metabolic features in mass spectrometry (MS)-based untargeted metabolomics. In this work, we proposed the concept of hypothetical neutral loss (HNL), which is the mass difference between a pair of fragment ions in a MS2 spectrum. We demonstrated that HNL values contain core structural information that can be used to accurately assess the structural similarity between two MS2 spectra. We then developed the Core Structure-based Search (CSS) algorithm based on HNL values...