Objective To investigate the surgical technique and effectiveness of anatomical posterolateral complex (PLC) reconstruction in treating severe posterolateral knee instability using Y-shaped allogeneic Achilles tendon. Methods The clinical data of 12 patients with Fanelli type C chronic PLC injury with severe posterolateral knee instability who met the selection criteria between June 2013 and August 2017 were retrospectively analyzed. There were 9 males and 3 females, with a median age of 34.5 years (range, 18-57 years) at the time of surgery. The average time from injury to surgery was 10.5 months (range, 3-24 months). All of them were complex ligament injuries and were treated with anatomical reconstruction of Y-shaped allogeneic Achilles tendon. The posterior and varus stress X-ray films were used to measure and calculate the difference of posterior displacement of tibia and difference of lateral joint opening of both knees to evaluate the backward stability of LCL and knee joint; the knee flexion 30° tibial external rotation test was used to calculate the difference of tibial external rotation angle of both knees to evaluate the stability of knee external rotation. The knee function was evaluated by the International Knee Documentation Committee (IKDC) 2000 subjective and objective scores. Results The operation successfully completed in 12 patients, and there was no vascular and nerve injury during the operation. The operation time was 2.5-3.5 hours, with an average of 2.7 hours; the intraoperative blood loss was 20-100 mL, with an average of 55 mL. All patients were followed up 15-41 months, with an average of 28.9 months. At last follow-up, the difference of posterior displacement of tibia, the difference of lateral joint opening, the difference of tibial external rotation angle of both knees, and the IKDC2000 subjective score, the objective scores of tibial external rotation and knee varus were significantly improved when compared with those before operation (P<0.05). One patient with anterior cruciate ligament+posterior cruciate ligament+PLC reconstruction found that the knee joint was stiff at 15 months after operation, and the range of motion of the knee joint was 10°-80°. After arthroscopic release, the range of motion of the knee joint was 5°-120°, the reconstructed ligament was stable. In the other patients, the knee flexion angle was normal in 2 cases; 9 cases had knee flexion limitation of 5°-10°, with an average of 6.4°; no knee extension was limited. Conclusion Anatomical PLC reconstruction using Y-shaped allogeneic Achilles tendon can effectively treat Fanelli type C chronic PLC injury with severe posterolateral knee instability and improve the knee joint stability.