<p>Due to the smooth and chemically inert surface of poly(p-phenylenebenzobisoxazole) (PBO) fibers, leading to poor interfacial properties in epoxy resin, which severely limits their applications in composite materials. The vast majority of research on PBO fiber interfaces is focused on PBO-HM type, while there is little research on PBO-AS type. Here, a modification strategy of in-situ regeneration of cellulose on PBO-AS fiber surface to enhance its interfacial properties in epoxy resin was proposed for the first time. First, PBO-AS fibers were oxidized and then underwent acyl chlorination, followed by chemical bonding with cellulose molecules. Second, cellulose molecules were in-situ regenerated on the fiber surface in different regeneration solvents. The results show that cellulose molecules were successfully in-situ regenerated on the fiber surface. Due to the physical and chemical properties of PBO-AS itself, the modification process inevitably damaged its mechanical properties, especially the oxidation process. In addition, the oxidation step slightly enhanced its interfacial strength in epoxy resin, while the rougher surface formed by cellulose regeneration was the main reason for the improvement of interfacial performance; the IFSS increased by 38.87% compared with untreated ones.</p>