The effect of circumference and displacement of the third fracture fragment on fracture healing after intramedullary nailing of femoral shaft fractures with a third fracture fragment was investigated. A retrospective cohort study was conducted to analyze the data of 142 patients who suffered femoral shaft fractures with a third fracture fragment and were admitted to the First People's Hospital of Lianyungang from February 2016 to December 2021. According to the circumference of the third fracture fragments, these were divided into three types of type 1: 71 cases; type 2: 52 cases; and type 3: 19 cases. On the basis of the diaphyseal diameter, the degree of displacement of the third fracture fragment was classified into three degrees of degree I: 95 cases; degree II: 31 cases; and degree III: 16 cases. Postoperative follow-up was performed to compare the fracture healing rate, healing time, and the modified Radiographic Union Scale for Tibia (mRUST) at 9th month after surgery in each group. All 142 patients were followed up after operation, with an average of (14.7 ± 4.1) months, and the overall healing rate was 73.4%. When the third fracture fragments were displaced in degree II and III, the mRUST score at 9th month in the type 1 group was higher than that in the type 2 and 3 groups (P = 0.017). Logistic regression analysis showed that greater displacement of third fracture fragments and greater circumference were associated with lower fracture healing rates (P < 0.05). After intramedullary nailing of femoral fractures, the degree of third fragment displacement and circumference affect fracture healing, and the former has a greater impact. When the third fracture fragment is displaced to degree II or III and its circumference is type 2 or type 3, it significantly affects the fracture healing. Intraoperative intervention to reduce the distance of third displacement of the fragment is required to reduce the incidence of non-union.