The total knee arthroplasty has been an important solution to treat the knee osteoarthritis and rheumatoid arthritis. However, a stable TKA at 90° of flexion and 0° of extension were given but significant cohort of patients have been found to have continued pain , swelling and sensation of instability during flexion and the mid-flexion instability is one of reasons. Mid-flexion instability can induce the failure of the total knee arthroplasty and ever the early revision knee surgery. At present ,the studys about the mid-flexion instability after the total knee arthroplasty are mainly on the theory, rarely in clinic .The aims of this article is to review thedefinition,clinical feature ,solutions,etiology and pathogenesis,et al.