Objective To study the effect of complete reduction on the clinical efficacy and sagittal alignment of patients with single segment L_4/5 degenerative lumbar spondylolisthesis undergoing posterior lumbar interbody fusion. Methods A total of 70 patients who underwent posterior lumbar interbody fusion for L_4/5 lumbar degenerative spondylolisthesis in our hospital from Jan. 2015 to Jan. 2018 were included. They were divided into complete reduction group and partial reduction group, with 35 cases in each group. The general data, operation parameters, radiographical parameters and clinical efficacy of the 2 groups were analyzed. Results There were no significance in gender, age, body mass index, spondylolisthesis degree, or preoperative thoracic kyphosis (TK), lumbar lodorsis (LL), pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), sagittal vertical axis (SVA), visual analogue scale (VAS) score, or Oswestry disability index (ODI) score between the 2 groups (all P>0.05). Compared with the partial reduction group, the complete reduction group had more intraoperative blood loss ([230.1±67.6] mL vs[182.5±56.3] mL), longer operative time ([176.4±35.2] min vs[152.6±23.4] min), and higher correction rate ([89.5±10.2]% vs[30.5±20.5]%) (all P<0.001). Compared with those before operation, TK, PT and SVA in the 2 groups were decreased, while LL was increased. At 3 months after operation, SVA in the complete reduction group was smaller than that in the partial reduction group ([26.5±21.2] mm vs[34.5±24.6] mm, P=0.012). At the last follow-up, LL, PT and SVA in the complete reduction group were smaller than those in the partial reduction group ([45.3±5.9] ° vs[48.2±6.3] °,[16.4±6.8] ° vs[18.3±7.1] °,[26.8±23.2] mm vs[36.5±26.2] mm, all P<0.05); and the VAS and ODI scores in the complete reduction group were lower than those in the partial reduction group (1.2±0.5 vs 2.2±0.8, 14.5±4.5 vs 20.9±6.4, both P<0.05). Conclusion As for patients with L_4/5 single segment degenerative lumbar spondylolisthesis, either complete reduction or partial reduction can achieve better clinical efficacy. Compared with partial reduction, patients with complete reduction have more blood loss and longer operative time, but they have better long-term clinical efficacy and can restore a better sagittal profile.