Abstract: The abuse of new psychoactive substances (NPS) has been increasing dramatic worldwide since late 2000s. By the end of 2019, more than 900 NPS had been reported to the United Nations Office for Drugs and Crime (UNODC). NPS can be subdivided in different groups, such as synthetic cannabinoids, synthetic cathinones, or novel synthetic opioids (NSOs). Although the total number of emerging NPS has slowly decreased in recent years, more and more NSOs have appeared on the illicit drugs market. According to the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) European Drug Report 2017, NSOs have been involved in overdose drug related deaths in some parts of Europe and North America, these compounds have been found in 82% of fatal overdoses. This phenomenon, related to the misuse of prescription opioids, has been dubbed an “opioids crisis”. Among NSOs are U-47700, U-49900, or U-48800, and so on, which belong to the so-called U-drugs and are structurally non-related to classical opioids such as morphine and fentanyl. U-drugs substances were originally synthesized in the 1970s and 1980s by the Upjohn Company, a pharmaceutical company from Kalamazoo, MI, USA, and appeared on the illicit drug market since 2014. Since U-47700 was internationally controlled in 2017, compounds with similar chemical structures, adapted from the original patented compounds, have emerged across the illicit synthetic drugs market. An increasing number of deaths have been associated with the use of U-drugs novel synthetic opioids, particularly in North America. In 2015-2019, a series of six U-drugs novel synthetic opioids were identified by means of ultra performance liquid chromatography-quadrupole time-of-flight-mass spectrometry (UPLC-QTOF MS), gas chromatography-mass spectrometry (GC-MS), and nuclear magnetic resonance spectroscopy (NMR) in China. The mass-spectral fragmentations of these compounds following electron ionization (EI) and electrospray ionization (ESI) under collision-induced dissociation (CID) mode were studied. Compounds were analyzed on Aglient DB-5 MS column (30 m×0.25 mm×0.25 μm) with an initial temperature of 140 ℃ for gradient increased temperature by GC-MS, and Acquity UPLC BEH C18 column (2.1 mm×100 m×1.7 μm) with 0.1% formic acid aqueous solution (A)-acetonitrile (B) as mobile phase for gradient elution by UPLC-QTOF MS. According to the structure and the typical fragmentations, these compounds could be easily identified. The countless possibilities to create U-drugs novel synthetic opioids by small changes in chemical structures pose a growing challenge to forensic analysts. So this work will be helpful to assist forensic laboratories in identifying these kinds of compounds or other substances with similar structure in case work.