Objective:To study the correlation between non-contact anterior cruciate ligament (ACL) injury and functional ankle instability (FAI) in young patients.Methods:A retrospective analysis was conducted of the 102 patients with non-contact ACL injury[61 males and 41 females, with an age of (31.9±6.1) years and a Tegner activity score of (6.1±1.9) points] who had been treated at Department of Orthopedics, Sun Yat-sen Memorial Hospital from January 2017 to March 2020 (injury group). Another 102 citizens without ACL injury from Guangzhou [56 males and 46 females, with an age of (30.3±7.2) years and a Tegner activity score of (6.0±2.1) points] were recruited as a control group. The Cumberland ankle instability tool (CAIT) and the Ankle Joint Functional Assessment Tool (AJFAT) were used to assess whether the subjects had self-conscious FAI or not. A correlation analysis was conducted using the data collected.Results:The 2 groups were comparable because there were no significant differences between them in general data ( P>0.05). By the CAIT score, the incidence of FAI in the injury group [52.9% (54/102)] was significantly higher than that in the control group [32.4% (33/102)] ( P<0.05); by the AJFAT score, the incidence of FAI in the injury group [59.8% (61/102) ] was significantly higher than that in the control group [39.2% (40/102)] ( P<0.05). Pearson correlation analysis showed that diagnoses of FAI by CAIT and by AJFAT were respectively correlated with ACL injury ( r=-0.159, P=0.023; r=-0.215, P=0.002). Conclusions:The incidence of FAI may be high in patients with ACL injury and there is a correlation between FAI and ACL injury.