Background Posterior Instrumented Transforaminal Lumbar Interbody Fusion (TLIF) is used to treat spinal stenosis. Minimally invasive surgery (MIS) can cause less muscle injury than conventional open surgery (COS) The purpose of this study was to compare the degree of postoperative fatty degeneration in the paraspinal muscles and the spinal decompression between COS and MIS based on magnetic resonance imaging (MRI). Methods Forty-six patients received TLIF (21 COS, 25 MIS) from February 2016 to January 2017. Lumbar MRI was performed within 3 months before surgery and 1 year after surgery. The postoperative muscle-fat-index (MFI) change of the paraspinal muscles (multifidus and erector spinae) and the dural sac cross-sectional area (DSCAS) change were compared between the 2 groups. Results The average MFI change at L2-S1 erector spinae muscle was significantly greater in the COS group (27.37 ± 21.37% vs. 14.13 ± 19.19%, P = 0.044). A significant MFI change difference between the COS and MIS group was also found in the erector spinae muscle at the caudal adjacent level (54.47 ± 37.95% vs. 23.60 ± 31.59%, P = 0.016). No significant differences at the operated and cranial adjacent level were found. DSCSA improvement after surgery was significantly greater in the COS group (128.15 ± 39.83 mm2 vs. 78.15 ± 38.5 mm2, P = 0.0005) Conclusion COS is associated with more prominent fatty degeneration of the paraspinal muscles. Statically significant post-operative MFI change was only noted in erector spinae muscle at caudal adjacent level and L2-S1 mean global level. COS produces a greater decompressive effect than MIS.