Objective To develop the finite element model of six-year-old child occupant lower extremity with higher biofidelity and validate the model of knee joints, as well as analyze the biomechanical responses of growth plate under frontal impact load and injury mechanisms of the knee joint by using this model. Methods The six-year-old child occupant lower extremity with growth plate was modeled based on children’s anatomy and CT images, and corresponding material properties of the lower extremity model were assigned．The model was validated according to biomechanical experiments by Kerrigan et al. and Haut et al. and then was used to analyze the injury results of growth plate with different material properties. Results The model validation was qualified by comparing the curves from the experimental and simulation results．The growth plates at knee regions could change injury patterns of the child occupant lower extremity fracture. The material properties of growth plate could affect threshold of axial damage of the femur as well as relative position of the fracture． Conclusions The validated model can be used for related study and application on biomechanical responses and injury mechanisms of six-year-old child occupant lower extremities．