Objective To explore the effectiveness of thumb blocking technique through closed reduction of ulnar Kirschner wire threading in the treatment of Gartland type Ⅲ supracondylar humerus fractures in children. MethodsThe clinical data of 58 children with Gartland type Ⅲ supracondylar humerus fractures treated with closed reduction of ulnar Kirschner wire threading by thumb blocking technique between January 2020 and May 2021 were retrospectively analyzed. There were 31 males and 27 females with an average age of 6.4 years ranging from 2 to 14 years. The causes of injury were falling in 47 cases and sports injury in 11 cases. The time from injury to operation ranged from 24.4 to 70.6 hours, with an average of 49.6 hours. The twitch of ring and little fingers was observed during operation, the injury of ulnar nerve was observed after operation, and the healing time of fracture was recorded. At last follow-up, the effectiveness was evaluated by Flynn elbow score, and the complications were observed. Results There was no twitch of the ring and little fingers when the Kirschner wire was inserted on the ulnar side during operation, and the ulnar nerve was not injured. All children were followed up 6-24 months, with an average of 12.9 months. One child had postoperative infection in the operation area, local skin redness and swelling, and purulent secretion exudation at the eye of the Kirschner wire, which was improved after intravenous infusion and regular dressing change in the outpatient department, and the Kirschner wire was removed after the initial healing of the fracture; 2 children had irritation at the end of the Kirchner wire, and recovered after oral antibiotics and dressing change in the outpatient department. There was no serious complication such as nonunion and malunion, and the fracture healing time ranged from 4 to 6 weeks, with an average of 4.2 weeks. At last follow-up, the effectiveness was evaluated by Flynn elbow score, which was excellent in 52 cases, good in 4 cases, and fair in 2 cases, and the excellent and good rate was 96.6%. ConclusionThe treatment of Gartland type Ⅲ supracondylar humerus fractures in children by closed reduction and ulnar Kirschner wire fixation assisted with thumb blocking technique is safe and stable, and will not cause iatrogenic ulnar nerve injury.