[Objective] This study was aimed at investigating the causes of lower extremity weaknesses after posterior spine fusion surgery, and looking at subsequent treatment strategies. [Methods] Patients who underwent posterior lumber spine fusion surgery in the Peking University First Hospital between Jan 2009 and Dec 2018 were counted. Those who needed secondary surgery because of subsequent lower extremity weaknesses were selected. CT scans and MRIs were used to evaluate the reasons for weaknesses before secondary surgery. Muscle strength was evaluated after surgery. [Results] Thirty patients (30/4078, 0.74%) required a secondary surgery because of lower extremity weaknesses after posterior lumber spine fusion surgery. The main causes of weakness were: internal fixation malposition and loosening (11 patients, 36%); epidural hematomas (9 patients, 30%); insufficient decompression (5 patients，17%) ; and nerve root edemas (5 patients，17%). Twenty-seven patients (90%) got improved muscle strength after their secondary surgery. [Conclusion] The main causes of weakness were internal fixation malposition and loosening, epidural hematomas, insufficient decompression or root edemas. There may be positive, therapeutic effects to subsequent, active surgical exploration.