Objective To analyze the diagnostic efficiencies of magnetic resonance imaging (MRI) and ultrasound for knee anterior cruciate ligament (ACL) injury comparatively. Methods A total of 110 patients with suspected ACL injury treated in the department of orthopedics of the hospital from June 2015 to June 2017 were collected retrospectively. All patients underwent MRI and ultrasonography before operation, and the imaging results were compared with arthroscopic findings. Results Among the 110 cases, 63 cases were ACL complete injury, 28 cases were partial injury, 5 cases of absorption or atrophy and 14 cases were normal confirmed by arthroscopy. With the arthroscopic results as the golden standard, The diagnostic accuracy, sensitivity, specificity, negative predictive value, positive predictive value and kappa value of MRI for ACL injury were 90.00%, 92.71%, 71.43%, 95.70%, 58.82% and 0.612, respectively, while those of ultrasound were 84.55%, 88.54%, 57.14%, 93.41%, 42.11% and 0.586, respectively (P>0.05). Conclusion Both of MRI and ultrasound are of high diagnostic value for ACL injury. Ultrasound has advantages of noninvasive, low cost and short examination time, and we can choose a suitable examination method according to the advantages and disadvantages of each examination method in clinical practice.