Abstract:[Objective] To compare the clinical efficacy of decompression by unilateral biportal endoscopy (UBE) versus extended posterior lumbar interbody fusion (PLIF) for adjacent spondylosis secondary to lumbar fusion. [Methods] From January 2021 to September 2022, 57 patients underwent surgical treatment for adjacent spondylosis after lumbar fusion. According to preoperative doctor-patient discussion, 31 patients received UBE decompression, while other 26 patients received extended PLIF. The documents regarding perioperative period and followup were compared between the two groups. [Results] All patients in both groups had corresponding surgical procedures performed smoothly, despite that 1 case in each group suffered from membrane tear of the outer nerve root with transient pain of lower extremity after surgery, which were treated by symptomatic treatment such as hormone and dehydration. The decompression group proved significantly superior to the extended fusion group in terms of operation time [(84.1±11.5) min vs (149.2±11.4) min, P<0.05], total length of incision [(2.3± 0.4) cm vs (12.3±1.5) cm, P<0.05], intraoperative blood loss [(27.9±4.0) ml vs (266.5±16.0) ml, P<0.05], intraoperative fluoroscopy [(3.1± 0.5) times vs (5.4±0.5) times, P<0.05], postoperative ambulation time [(2.5±0.5) days vs (3.6±0.6) days, P<0.05], hospital stay [(5.4±0.6) days vs (9.6±0.6) days, P<0.05], in addition, the former recovered full weight-bearing activity significantly earlier than the latter [(49.7±3.3) days vs (60.2±1.1) days, P<0.05]. The VAS for lumbago and leg pain, as well as ODI scores were significantly improved in both groups over time (P<0.05). In terms of imaging, the vertebral canal area and lateral recess sagittal diameter were significantly increased after surgery in both groups (P<0.05), whereas the lumbar lordosis remained unchanged remarkably in both of them (P>0.05). However, the decompression group was significantly less than the extended fusion group in terms of the spinal canal area [(160.3±6.6) mm2 vs (179.9±1.8) mm2 , P<0.05] and the lateral recess sagittal diameter [(3.9±.01) mm vs (4.2±0.1) mm, P<0.05] at the last follow-up. [Conclusion] The decompression under unilateral biportal endoscopy is a safe, feasible, less traumatic technique without destroy to the posterior tension band structure of the lumbar spine, does get fast recovery, and good clinical consequence.