Objective To investigate the effectiveness of lateral decubitus position assisted plate internal fixation through a lateral incision to assist reduction combined with intramedullary nail in the treatment of complicated subtrochanteric femoral fracture. Methods The clinical data of 16 patients with complicated subtrochanteric femoral fractures (Seinsheimer type Ⅲ-Ⅴ) treated with lateral decubitus position assisted plate internal fixation through a lateral incision to assist reduction combined with intramedullary nail between September 2017 and August 2020 were retrospectively analyzed. There were 13 males and 3 females with an average age of 47 years (range, 26-85 years). There were 12 cases of high-energy injury and 4 cases of low-energy injury. According to Seinsheimer classification, there were 3 cases of type ⅢA, 2 cases of type ⅢB, 7 cases of type Ⅳ, and 4 cases of type Ⅴ. The time from injury to operation ranged from 2 to 6 days, with an average of 4.7 days. The operation time, intraoperative blood loss, postoperative drainage volume, hospitalization stay, surgical complications, fracture healing time, and collodiaphyseal angle of the affected and healthy sides before and after operation were recorded. Hip fracture Harris score was used to evaluate hip function. Results The operation time was 90-180 minutes (mean, 135.9 minutes), the intraoperative blood loss was 200-400 mL (mean, 288.8 mL), the postoperative drainage volume was 120-220 mL (mean, 140.0 mL), and the hospitalization stay was 12-22 days (mean, 15.8 days). All the 16 patients were followed up 9-12 months (mean, 9.9 months). There was 1 case of incision superficial infection after operation, which healed after anti-infection treatment; no complication such as deep venous thrombosis of lower limbs, coxa vara deformity, re-fracture, or broken nails occurred. All the fractures healed successfully, the healing time ranged from 12 to 20 weeks, with an average of 17.5 weeks. At 6 months after operation, the Harris score was 87-96, with an average of 91.5; the results were excellent in 11 cases and good in 5 cases, with the excellent and good rate of 100%. The collodiaphyseal angle of the affected side was (124.0±5.7)°, while that of the healthy side was (132.0±2.1)°, showing significant difference between the two sides (t=–7.376, P=0.001). At last follow-up, the collodiaphyseal angle of the affected side was (129.0±3.2)°, which significantly improved when compared with that before operation (t=–6.175, P=0.002), and there was no significant difference between the affected side and the healthy side (t=–2.648, P=0.181). ConclusionLateral decubitus position assisted plate internal fixation through a lateral incision to assist reduction combined with intramedullary nail is a reliable internal fixation method for the treatment of complicated subtrochanteric femoral fractures. The use of plate reduction is conducive to maintaining the force line of the femoral trochanter. The enlargement of the incision is conducive to the accurate implantation of intramedullary nails without affecting fracture healing.