Objective To investigate whether the geometric triangular relationship between the Blumensaat line and the tibial plateau can serve as an auxiliary diagnostic index for identifying and evaluating anterior cruciate ligament （ACL） injuries， thereby enhancing diagnostic accuracy. Methods A retrospective self?controlled study was conducted involving 64 patients diagnosed with unilateral ACL injury via arthroscopy and treated with ACL reconstruction at Qingdao Municipal Hospital from January to August 2023. Ultimately， 40 cases were included in the analysis. Preoperative MRI images of the affected knees and contralateral healthy knees were collected as the ACL injury group and control group， respectively. In the MRI images， point C， point D， and point E were defined as the anterior end， posterior end， and midpoint of the Blumensaat line， respectively； point A and point B were defined as the turning points of the anterior and posterior edges of the tibial plateau， respectively； and point C' was defined as the intersection of the extension line of DC and line AB. Based on these anatomical landmarks， angles ∠DAB， ∠CEB， ∠DC'B， and ∠CDB were established. The intra?class correlation coefficient （ICC） was calculated to assess measurement consistency and reproducibility. Differences between the aforementioned angles were compared， and the area under the receiver operating characteristic （ROC） curve was computed to evaluate diagnostic performance. Results For ∠CDB， ∠DC'B， ∠CEB， and ∠DAB， the intra?group and inter?group correlation coefficients all exceeded 0.80， indicating excellent consistency and reproducibility. Compared to the control group， the angles ∠CDB， ∠CEB， and ∠DAB in the ACL injury group were significantly reduced （P < 0.001）. Among these， ∠DAB appears to be the most reliable index for diagnosing and evaluating ACL injuries， with an area under the receiver operating characteristic curve （AUC） of 0.829， a cut?off value of 42.2°， a sensitivity of 82.5%， and a specificity of 80.0%. Conclusion The geometric triangular relationship between the Blumensaat line and the tibial plateau in MRI images， particularly the angle ∠DAB， can serve as an auxiliary indicator for diagnosing and evaluating ACL injuries， thereby enhancing diagnostic accuracy.