Background Magnetic resonance imaging (MRI) is the most sensitive technique for evaluating the healing process and should be performed before the patients return to their exercise routines. The aim of this research was to diagnose chronic lumbago associated with lumbar muscle strain and to monitor healing process by MRI. Methods Sixty-five symptomatic cases of chronic lumbago caused by lumbar muscle strain were collected from March 2009 to October 2011. MRI was used to examine, diagnose and monitor the healing process. The control group included 65 random cases of asymptomatic volunteers. MRI methods included routine sequences of GRE T1WI, TSE T2WI and special sequences of T2-STIR-FS, combined with DWI. We compared the MRI characteristics of symptomatic cases before and after healing and with asymptomatic controls. Results The important MRI characteristics of chronic lumbago with lumbar muscle strain included: (1) The low back muscle showed edema. (2) The low back intermuscular spaces showed edema and/or fluid. (3) The low back spaces beside the spinous process showed edema and/or fluid. (4) The low back vertebral articular process fossae or transverse process fossae showed fluid. Of these image characteristics, the intermuscular space edema provided the best diagnostic sensitivity, Se=83%, with Yl=0.63, n=74%. The low back muscle edema provided the best diagnostic specificity, Sp=100%, with Yl=0.66, pi=83%. And the spaces edema beside the spinous process provided the best diangnostic accuracy, pi=86%, with Yl = 0.71, Se=80%, Sp=91%. The diagnosis accurate could be improved by combining multiple MRI characteristics. The diagnostic accuracy could achieve pi=93%, with Yl=0.86, Se=100% and Sp=86% when two characteristics were combined. After rehabilitation care, the edema disappeared on the repeated MRI. Conclusions MRI may well be a useful diagnostic method for lumbago with lumbar muscle strain. Combining routine sequences with T2-STIR-FS and DWI sequences could demonstrate the pathological changes of lumbar muscle strain and monitor the healing.