Autophagy is a highly conserved cellular homeostasis maintenance mechanism in eukaryotes. Microtubule-associated protein light chain 3 (LC3) plays a crucial role in autophagy. It has multiple pairs of protein-protein interactions (PPIs) with other proteins, and these PPIs have an effect on the regulation of autophagosome formation and the recruitment of autophagic substrates. In our previous work, a small molecule covalent inhibitor DC-LC3in-D5 which could inhibit LC3A/B PPIs was identified, but a detailed study of structure-activity relationships (SARs) was lacking. Herein, a new molecule LC3in-C42 was discovered utilizing the hybridization of advantageous fragments, whose potency (IC50 = 7.6 nM) had been greatly improved compared with that of DC-LC3in-D5. LC3in-C42 inhibits autophagy at the cellular level and its efficacy far exceeds that of DC-LC3in-D5. Thus far, LC3in-C42 stands as the most potent LC3A/B small molecule inhibitor. LC3in-C42 could serve as a powerful tool for LC3A/B protein and autophagy research.