Objective To study the imaging performance and mechanism of acute patellofemoral joint impingent syndrome.Meth-ods The MR images and clinical data of acute patellofemoral joint impingent syndrome were analysed retrospectively in 10 patients, which were confirmed by clinic .MRI and DR examination were performed in all patients,in which 6 patients had complete knee ar-throscopy material.The imaging performance and mechanism of acute patellofemoral joint impingent syndrome were summarized re-spectively.Results All cases had clear history of buckling ectropion trauma;Patellar dislocation or subluxation were detected in 9 cases on DR examination,10 cases on MRI;Patellar medial fracture or osteochondral fracture caused by acute slipped patella was de-tected in 1 case on DR examination,2 cases (3 places)on MRI;Medial patellar retinaculum injury were detected in 10 cases on MRI,which were corresponding to that on surgery.Articular injury and subchondral marrow edema in particular parts of joint (an-terolateral of lateral femoral and medial patellar)were detected in 9 cases on MRI.MRI features of articular cartilage injury were confirmed by arthroscopy.MRI showed ACL or PCL injury in 3 cases,meniscus tear in 4 cases,which were quite consistent with the arthroscopy findings.Conclusion MRI can effective display joint cartilage damage,subchondral marrow edema in particular parts of the joint,which is useful to clinical prediction and treatment at early stage.