Abstract: By using the improved vacuum preloading model, the effects of grain size distribution on the improvement of dredger fill are studied. Laboratory vacuum preloading tests on seven groups of samples with different grain size distributions are performed under the same loading condition. Then axial settlement, drainage flow and physical mechanics are analyzed to study the effects of the grain size distribution on the improvement of dredger fill. The results show that different grain size distributions have great influences on the axial settlement, drainage flow and the properties of soils, and the grain size distribution is a key factor to influence the improvement of the dredger fill if the vacuum preloading is adopted to improve dredger fill. This study has guiding significance for choosing the proper improvement method for the dredger fill.