Objective To investigate the evaluation, diagnosis, and surgical treatment of knee joint diagonal lesion and to observe its effectiveness. Methods A retrospective analysis was performed on 9 patients with knee joint diagonal lesion admitted between November 2011 and November 2019, including 7 males and 2 females, aged 18-61 years, with an average age of 38.3 years. According to the clinical staging criteria of multiple ligament injury of knee joint, 6 patients with acute stage injury (≤3 weeks), the time from injury to operation was 5-16 days, with an average of 9.3 days; 3 patients with old stage injury (>3 weeks), the time from injury to operation was 7, 12, 144 months, respectively. Posterolateral complex injuries were all type C according to Fanelli classification, and the posterior cruciate ligament injury was grade 3 according to American Medical Association (AMA) classification. According to the three columns classification of tibial plateau fracture, there were 2 cases of zero column fracture and 7 cases of medial column fracture. Patients with acute injury were treated with posterior cruciate ligament reconstruction, posterolateral complex repair and/or reinforcement reconstruction, and tibial plateau fracture was treated with conservative treatment or internal fixation. Patients with malunion of tibial plateau in old stage were treated with limited osteotomy, bone grafting, and internal fixation; the stability of the knee joint was further evaluated during operation, then the posterior cruciate ligament reconstruction and/or posterolateral complex repair and reconstruction were performed as needed. Lysholm score and the International Knee Documentation Committee (IKDC) score before and after operation were used to evaluate knee function and effectiveness. Results All patients were followed up 24-119 months, with an average of 51.7 months. All the incisions healed by first intention, and no complication such as incision infection, deep venous thrombosis of lower extremities occurred, and there was no reoperation during follow-up. The full-length X-ray films of both lower extremities at 6 months and 1, 2 years after operation showed that the alignment of both lower extremities returned to normal, and the stress position X-ray films showed that the stability in all directions recovered well. The X-ray films of the knee joint at 6 months after operation showed that the fracture of the tibial plateau healed well. At 1 year after operation, MRI showed that the signal of the repaired and reconstructed ligament was good, and there was no reconstruction failure such as ligament absorption or rupture occurred. At 1 year after operation, the muscle strength of the affected limbs was all rated as grade Ⅴ, and the active and passive range of motion of the knee joint recovered to 0°-130°. At 2 years after operation, the Lysholm score and IKDC score significantly improved when compared with those before operation (P<0.05). Conclusion Knee joint diagonal lesion is a special type of posterior cruciate ligament and posterolateral complex injury with anteromedial split and collapse fracture of tibial plateau, which requires correct evaluation and diagnosis of the injured structure, and detailed surgical and rehabilitation programs to achieve better effectiveness.