Background <p>Free-floating venous thrombosis (FFVT), a distinct subtype of deep vein thrombosis (DVT), is associated with pulmonary thromboembolism (PTE) and carries a high mortality risk.</p> Objective <p>This study aimed to develop a nomogram to predict the prognosis of FFVT in patients with closed traumatic fractures.</p> Materials and methods <p>A retrospective analysis of clinical and ultrasound data from 326 patients with FFVT post-closed traumatic fractures was conducted. Patients were divided into training (<i>n</i> = 240, January 2019–June 2023) and validation (<i>n</i> = 86, June 2023–June 2024) sets. Prognostic risk factors were identified using LASSO and multivariable logistic regression. A nomogram was constructed using R Studio, and its predictive accuracy was validated via calibration curves, receiver operating characteristic (ROC) analysis, and external validation.</p> Results <p>Independent risk factors for FFVT progression to closed thrombus included D-dimer levels, FFVT location, collateral blood flow volume around the thrombus, and thrombus margins (<i>P</i> &lt; 0.05). The model demonstrated high discriminative ability, with a C-index of 0.945. ROC analysis revealed areas under the curve (AUC) of 0.949 (training set) and 0.924 (validation set). Calibration curves confirmed strong agreement between predicted and observed outcomes.</p> Conclusion <p>The nomogram provides an accurate prognostic tool for FFVT in patients with closed traumatic fractures, aiding clinical decision-making to improve patient outcomes.</p> Clinical trial number <p>Not applicable.</p>