BACKGROUND: Spontaneous psoas hematoma is a very rare clinical entity, and the pathogenesis and pathologic mechanisms of spontaneous psoas hematoma remain unclear, thus, it is of great value to explore. CASE PRESENTATION: We encountered a patient who developed femoral nerve paralysis due to psoas muscle hematoma following posterior lumbar fusion surgery. A 69-year-old female with lumbar spinal canal stenosis underwent posterior lumbar fusion at the L3-4 and L4-5 levels. On the 7th postoperative day, the patient complained of severe abdominal pain along with right limb pain and weakness. A computed tomography image showed a large acute psoas muscle hematoma on the right side. After conservative treatment, the psoas hematoma was resolved and the patient's femoral nerve paralysis gradually recovered. CONCLUSIONS: Spontaneous psoas hematoma is a rare and extremely dangerous complication after lumbar spine surgery. For high - risk patients who have taken anticoagulants before the operation, we must be vigilant against the occurrence of this complication.