Objective To improve the biological fidelity of the thorax flexible body in the original MADYMO child human model, so as to further study pediatric thorax injuries of child occupant. Methods A finite element model of the six-year-old pediatric thorax was built by adopting the method of reverse modeling based on CT images. By replacing the thorax model with flexible body in MADYMO six-year-old human model, an improved human model containing biomechanical thorax model was developed. The model was verified by joint validation of two tests, including Kroell’s adult chest impact experiment combined with Irwin and Mertz’s scaling method, and Jun Ouyang’s thoracic impact test on pediatric cadavers. Results The response of this established thorax model was in good agreement with scaling method and cadaver test data, and the thorax model was much more accurate than the original flexible body model. The resilience of simulation model was consistent with cadaver test. Conclusions The validity of the model is verified, and can be further used in occupant injury analysis in vehicle frontal crash.