Objective: To investigate the risk factors for secondary acute traumatic coagulopathy (ATC) in children with severe traumatic brain injury (sTBI), providing reference for early identification of the risk of ATC and development of related intervention strategies. Methods: A total of 115 children with sTBI were enrolled in this study. They were divided into the secondary ATC group (n = 38) and the non-secondary ATC group (n = 77) according to whether ATC occurred within 24 h post-injury. Coagulation parameters were dynamically monitored at 6, 12, and 24 h post-injury. Univariate and logistic regression analyses were used to identify the risk factors for secondary ATC, and a nomogram was constructed on this basis. Results: The incidence of secondary ATC among the 115 children with sTBI was 33.04% (38/115). Compared with the non-secondary ATC group, the secondary ATC group demonstrated significant increases in ISS score at admission, the proportions of children with hypothermia at admission, surgery within 24 h after admission, platelet count 2 mmol/L at admission, and a significantly decreased GCS score at admission (P < 0.05). Compared with levels at 6 h post-injury, APTT, PT, and D-D were increased, and FIB was decreased at 12 and 24 h post-injury only in the secondary ATC group (P < 0.05). Compared with the non-secondary ATC group, the secondary ATC group presented significant increases in APTT, PT, and D-D levels, and a significant decrease in FIB level (P < 0.05). Multivariate logistic regression analysis identified the following independent influencing factors for secondary ATC in children with sTBI: ISS score at admission (OR = 1.670, 95% CI: 1.126–2.477), lactate level at admission (OR = 1.340, 95% CI: 1.091–1.647), FIB at 12 h post-injury (OR = 0.695, 95% CI: 0.544–0.888), and D-D at 12 h post-injury (OR = 1.480, 95% CI: 1.067–2.053) (all P < 0.05). The nomogram model indicated that these four factors had high predictive value for the occurrence of secondary ATC. Conclusion: Higher injury severity score (ISS) and lactate level at admission, as well as lower FIB level and higher D-D level at 12 h post-injury, are independent risk factors for secondary ATC in children with sTBI. The nomogram prediction model constructed based on these four key indicators demonstrates good predictive efficacy, providing an intuitive and practical quantitative tool for early identification of children at high risk of ATC.