This report is a rare case of a lumbar interbody fusion cage dislocation in the procedure of Minimal Invasive Transforaminal Lumbar Interbody Fusion surgery (MI-TLIF) for spondylolisthesis of segment L5/S1. A 65-year-old lady underwent MI-TLIF surgery, and the interbody fusion cage was dislocated forward into retroperitoneal space. The cage was extraction with lateral approach and replaced by larger size of internal fixation devices. The patient felt pain, numbness, and unable to lift limbs after surgery, these symptoms were gradually relieved 3 months later. Spondylolisthesis of L5/S1 segment, incomplete reduction, bilateral implantation of bullet-shaped cages was all risk factors for dislocation of fusion cage. The cage which falls into retroperitoneal cavity and extraction by lateral approach could cause injury to lumbosacral trunk. Deciding extraction or not in a second stage after examination and evaluation may be a good idea.