Abstract: Objective To explore the influence of surgery at different opportune moment on postoperative knee joint function in patients with anterior cruciate ligament injury. Methods The clinical data of 76 patients with unilateral anterior cruciate ligament (ACL) injury treated in our hospital from January, 2012 to June, 2015 were analyzed, 35 patients who underwent surgery within 3 weeks after admission were assigned into the observation group, and 41 patients who underwent surgery over 3 weeks after admission were assigned into the control group. The range of motion, Tegner, Lysholm and IKDC scores of the two groups before and after the operation were compared. The anteroposterior distances of the tibia were measured by Kt2000 instrument before and after the operation, and the complications of both groups were followed up regularly, and the postoperative complications were compared. Results The difference in Tegner, Lysholm and IKDC score before and after the surgery in each group was not statistically significant (P>0.05). The activity, Tegner, Lysholm and IKDC scores in the observation group were significantly higher than those in the control group, the difference was statistically significant (P<0.05); the tibial anteroposterior movement distance by using Kt2000 knee joint arthrometers at 30° and 90° of flexion with a force of 133.32 N in both groups were significantly smaller than the preoperative level, the differences were statistically significant (P<0.05). After the operation, there was no significant difference in the tibial anteroposterior movement distance at 30° and 90° of flexion with a force of 133.32 N, (P>0.05). The incidence of postoperative complications in the control group was significantly higher than that in the observation group, the difference was statistically significant (1/35, 2.86% vs 5/41, 12.20%; log-rank χ²=3.891, P=0.048). Conclusion Anterior cruciate ligament reconstruction surgery can effectively protect the activity and stability of the knee joint patients within 3 weeks after injury, it can effectively avoid the occurrence of meniscus injury of knee joint and knee joint adhesion after operation.