Central sarcopenia is associated with the prognosis of various orthopedic surgeries in the elderly. This study aims to investigate its impact on the outcomes of single-segment lumbar fusion surgery in elderly patients. Retrospective analysis was conducted on 314 patients aged 60 to 80 who underwent single-segment posterior lumbar fusion surgery due to degenerative lumbar diseases. Patients were categorized into high psoas and L4 vertebral index (PLVI) and low PLVI groups according to the MRI-measured PLVI for central sarcopenia. Basic patient data, surgery-related parameters, functional assessments at preoperative and postoperative 3, 6, and 12 months, and X-ray-based fusion status were compared. The basic data of the two groups showed no significant differences. Parameters including the operative segment, preoperative hemoglobin levels, surgical duration, and intraoperative blood loss exhibited no significant variances. However, notable differences were observed in postoperative initial hemoglobin levels, transfusion requirements, and length of hospital stay between the two groups. During the postoperative follow-ups at 3, 6, and 12 months, the VAS scores for lower back pain and ODI scores in the lower PLVI group were significantly higher compared to the high PLVI group. Additionally, the EuroQoL 5D scores were notably lower in the low PLVI group. There were no significant differences between the groups in terms of leg pain VAS scores at each time point and the fusion status at 12 months postoperatively. MRI-based central sarcopenia has a negative impact on the therapeutic effectiveness following single-segment lumbar fusion surgery in elderly patients.