Objective: To investigate the long-term outcomes of posterior long segment instrumentation and fusion in adult degenerative scoliosis (ADS), and to explore the correlative factors of sagittal imbalance after long segment instrumentation and fusion, and to summarize the impacts of different lowest instrumented vertebra (LIV) on sagittal complication rates. Methods: ADS patients who underwent long segment instrumentation and fusion between January 2008 and January 2014 were retrospectively reviewed and divided into two groups depended on LIV (L ₅ group and S₁ group). The follow-up time was at least five years. Radiographic parameters, Visual Analogue Score (VAS), Oswestry Disability Indexscore(ODI), the 12-item Short Form Health Survey-Physical Component Summary (SF-12 PCS)and sagittal complications were analyzed. And the correlation between sagittal imbalance and sagittal complications were calculated. Patients were divided into two groups(sagittal imbalance group and sagittal balance group) to explore the correlative factors of sagittal imbalance. Results: All of 56 patients were included and evaluated in this study, and 35 cases stopped at L ₅, 21 cases at S₁. The mean follow-up time was 6.6±1.6 years. No significant difference were found in age, gender, follow-up time and surgery method (P>0.05) between two groups. No significant differences were found in Cobb angle(11.8°±7.3° vs 12.6°±6.4°), coronal balance distance(CBD) (0.7±0.6 cm vs 1.0±0.5 cm), sagittal vertebral axis(SVA)(3.0±3.1 cm vs 2.6±5.6 cm), thoracic kyphosis(TK) (21.4°±9.6° vs 22.5°±6.9°), lumbar lordosis(LL) (-32.7°±7.3° vs-34.8°±9.1°), pelvic tilt (PT) (23.4°±9.8° vs 23.3°±8.1°), VAS score(2.0±1.2 vs. 2.2±0.9), ODI score(18.5%±6.4% vs 19.3%±5.2%) and SF-12 PCSscore(45.5±5.2 vs 42.4±7.0) between two groups at final follow-up time(P>0.05). Radiographic parameters including Cobb angle, CBD, SVA and health related quality of life scores including VAS score, ODI score and SF-12 PCS scores were improved significantly at two weeks postoperatively, two-year follow-up time andfinal follow-up time in two groupscompared with those preoperatively (P<0.05). And no significant difference were found in radiographic parameters and health related quality of life scores between those at two years follow-up time and those at final follow-up time; Sagittal complication rates at final follow-up were higher than these at two years follow-up (82.1% vs 33.9%, P=0.000). Sagittal imbalance was related to internal fixation failure in L ₅ group (r=0.691, P=0.011), internal fixation failure in S₁ group (r=0.691, P=0.001) and pseudoarticulation formation (r=0.499, P=0.021). 10 patients in the sagittal imbalance group were with preoperative sagittal imbalance, 9 with preoperative coronal imbalance, 6 with internal fixation failure, 5 with proximal junctional kyphosis (PJK), 4 with adjacent segment degeneration during the follow-up periods, the incidence rates of which were higher than those in the sagittal balance group. LL in the sagittal imbalance group was lower than that in the sagittal balance group at two years follow-up time and final follow-up time. Compared with the sagittal imbalance group, the sagittal balance group achieved a higher SF-12 PCS (45.4±5.1 vs 41.6±7.4, P<0.05) and lower ODI scores (17.8%±6.1% vs 21.5±4.6%, P<0.05), and no significant difference was found in VAS scores between two groups (1.9±1.2 vs 2.4±0.8, P>0.05). The incidence of adjacent segment degeneration in L ₅ group was higher than that in S₁ group (P<0.05) at final follow-up time, and the incidence of pseudojoint in S₁ group was higher than that in L ₅ group (P=0.002) at final follow-up time. Conclusion Posterior long segment instrumentation and fusion for adult degenerative spinal deformity can obtain satisfying long-term clinical outcomes, and postoperative sagittal imbalance was related to preoperative sagittal/coronal imbalance, postoperativeinternal fixation failure, PJK, pseudoarticulation formation and adjacent segment degeneration. Distal fusion at L ₅ or S₁ may lead to high risk of adjacent segment degeneration or pseudoarticulation formation respectively.