To study the influence of differential subgrade settlement on the ballasted track settlement,a 1∶1 model test was designed.The differential subgrade settlement was simulated by designed void,and the dynamic train load was simulated by Centrifugal Exciter,then a study was made of the influence of differential subgrade settlement on the settlement of ballasted track before and after the sleeper is unsupported on the condition that the the settlement of ballasted track was stabilized.Experiment result shows that the curve of track settlement caused by differential subgrade settlement can be described by cosine function.When the area of differential subgrade settlement curve is small and doesn’t cause the sleeper to be unsupported,the ratio of the area of track settlement curve to differential subgrade settlement is 1.Based on those results,the calculation formula which is about ballasted track settlement caused by differential subgrade settlement is put forward,then the main causing factors are discussed.With the developing of differential subgrade settlement,once the ratio of the area of track settlement curve to the area of differential subgrade settlement is less than 1,the phenomenon that the sleeper is unsupported will appear and the ratio of area will decrease as the increase of differential subgrade settlement.