Methods: In total, 306 patients who received L5 floating lumbar fusion were included in this study. Clinical L5–S1 ASD was defined as newly developed radiculopathy in relation to the L5–S1 segment. Patients’ medical records and imaging data were retrospectively analyzed. The risk factors for clinical ASD were assessed by an inverse probability of treatment weighting-adjusted logistic regression analysis. Results: Clinical L5–S1 ASD occurred in 17 patients (5.6%). The mean onset time of L5–S1 ASD was 12.9±7.5 months after the primary surgery. Among these patients, 10 (58.8%) presented with clinical L5–S1 ASD within 12 months. Reoperation was performed in three patients (1.0%). The severity of L5–S1 disk degeneration did not affect the occurrence of L5–S1 ASD. Logistic regression analysis showed that the number of fusion levels was a significant risk factor for clinical L5–S1 ASD. Conclusions The incidence and characteristics of clinical L5–S1 ASD after L5 floating lumbar fusion were retrospectively investigated. This study established that the number of fusion levels was a significant candidate factor for clinical L5–S1 ASD. Careful clinical follow-up is deemed necessary after L5 floating lumbar fusion surgery, especially for patients who received multiple-level fusions.