Objective To research the diagnostic value of MRI signs of anterior cruciate ligament injury. Methods MRI images of 150 knee joints were retrospectively evaluated. Two radiologists evaluated all these images for the presence of 5 direct signs and 10 indirect signs. 150 patients underwent knee arthroscopy, compared evaluation results and arthroscopic results. Results Among the 5 direct signs, discontinuity of ACL and abnormal contour of ACL had relative higher sensitivity and specificity (P<0.05); Most of the indirect signs,had relative high specificity but low sensitivity. The following 5 indirect signs, bare area at the femoral origin of the ACL, Notch sign, uncovered posterior horn of lateral meniscus sign, PCL curvature ratio, and entire length of the PCL observed on a single coronal image, had relative higher sensitivity (P<0.05). Conclusion Direct signs were the main clues for diagnosing ACL in juries, and discontinuity and abnormal contour of ACL were the two most valuable signs of a tear. Indirect signs were only for assistant diagnosis and bare area at the femoral origin of the ACL, Notch sign, uncovered posterior horn of lateral meniscus sign, PCL curvature ratio, and entire length of the PCL observed on a single coronal image were the best predictors of an ACL tear．