Objective To compare the early clinical effects of unilateral biportal endoscopic lumbar interbody fusion (ULIF) and minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF)on single-segment lumbar stenosis with instability. Methods The patients who had single-segment lumbar spinal stenosis with instability and were treated in our hospital from August 2020 to May 2021 were selected.According to the operation methods,they were classified into ULIF group and MIS-TLIF group.The operation duration,hospital stay after operation,perioperative blood loss (drainage volume 48 h after operation,total blood loss),creatine kinase,inflammatory cytokines (C-reactive protein,interleukin-6),D-dimer,and the incidence of lower-extremity venous thrombosis were compared between the two groups.The visual analogue scale and Oswestry disability index were used to evaluate the functional recovery of the two groups in 1 week,1 month,and 3 months after operation. Results The ULIF group had longer operation duration (P<0.001) and shorter hospital stay after operation (P=0.022)than the MIS-TLIF group.The drainage volume 48 h after operation and total blood loss in ULIF group were lower than those in MIS-TLIF group (all P<0.001).The levels of creatine kinase (all P<0.001),C-reactive protein (P<0.001,P=0.002),and interleukin-6 (P=0.003,P<0.001) in ULIF group were lower than those in MIS-TLIF group on the 1st and 3rd day after operation.However,the D-dimer in ULIF group was insignificantly different from that in MIS-TLIF group on the 1st and 3rd day after operation (P=0.117,P=0.683).Lower-extremity venous thrombosis occurred in neither group.The score of visual analogue scale showed no significant difference between the two groups 1 week,1 month,and 3 months after operation (P=0.447,P=0.578,P=0.538),so did the Oswestry disability index (P=0.832,P=0.797,P=0.619). Conclusion ULIF shows similar clinical effect on single-segment lumbar stenosis with instability to MIS-TLIF,which features less bleeding,mild inflammation,mild muscle injury,but long operation duration.