Abstract: The influence of the vibration amplitude on the motion-induced aerodynamic force of the streamline box girder has been studied. Based on a 1:70 rigid sectional model and electronic pressure scanner valves, the aerodynamic force of the model has been obtained by wind tunnel tests. The range of the torsional amplitude is from 2° to 16°, the range of the vertical amplitude is from 5mm to 23mm, and the angles of attack are 0° and ±5°. At 0° angle of attack, when the torsional amplitude is less than 8° and the vertical reduced amplitude is less than 0.46, the proportions of the linear motion-induced aerodynamic force remain stable and are more than 95%. When the amplitude of the torsional vibration is greater than 8° and the angle of attack is +5°, the linear harmonic components decrease and the high-order harmonic ones increase with the increase of the amplitude. The change of flutter derivatives shows that the torsional amplitude affects A₂^*, A₃^* and H₂^* dramatically, which implies the nonlinear change of the aerodynamic force. However, the vertical amplitude only affects H₄^*, which is not the control parameter of flutter analysis, and has no influence on the other flutter derivatives. The brief conclusion is that although the force-amplitude behavior of a streamline box girder is nonlinear, there is no obvious high-order component in the motion-induced aerodynamic force under a certain amplitude, especially under 8° torsional amplitude.