Objective: To study the correlation between activated partial thromboplastin time (APTT) mixing test results and the inhibitor titers in hemophilia A inhibitor-positive patients. Methods: In this cross-sectional study, 41 patients with severe hemophilia A and inhibitors (and negative for lupus anticoagulant) were included from the hemophilia clinic of Shandong Blood Center from February 2022 to February 2024. All patients underwent APTT mixing test. The Rosner's index (RI, including the immediate RI and the RI after 2-hour water bath incubation [water bath 2h RI]), the time-dependent difference (Δ value), and the corrected percentage were calculated based on results of APTT mixing test. The median (interquartile range) of the corresponding indexes were calculated, and the ROC curves for identification of high inhibitor titers using the four indexes (the immediate RI, the water bath 2h RI, the Δ value, and the corrected percentage) were plotted, The correlations between APTT mixing test and inhibitor titers for coagulation factor Ⅷ (Factor Ⅷ, FⅧ) were investigated. Results: The median (lower quartile, upper quartile) of immediate RI, water bath 2h RI, Δ-value and corrected percentage for FⅧ inhibitor positive patients were 11.0 (5.4, 29.3)%, 45.0 (25.7, 75.0)%, 26.2 (7.6, 41.8) s, and 82.2 (58.5, 91.6)%, respectively. The median (lower quartile, upper quartile) of the immediate RI, water bath 2h RI, Δ-value and corrected percentage were 25.2 (13.0, 37.5)%, 64.1 (44.6, 72.6)%, 38.0 (14.3, 38.3) s, and 66.5 (50.1, 82.1)% for the high-titer inhibitor group, and 5.2 (4.2, 9.4)%, 17.9 (8.8, 28.0)%, 13.0 (7.6, 25.4) s, and 92.3 (88.0, 94.3)% for the low-titer inhibitor group. The AUCs of the ROC curves for discrimination between high and low titer inhibitor were: 0.9105 for immediate RI, 0.9118 for water bath 2h RI, 0.8873 for correcter percentage, and 0.6532 for Δ-value. Conclusion: High-titer inhibitors can be highly suspected in hemophiliac patients with an immediate RI >10% and a water bath 2h RI >45%, and the presence of low-titer inhibitors is suspected in patients with a 4-second < immediate RI <10% and a 13% < water bath 2h RI <45%.