A vacuum generator system has been newly developed to implement upward flow and simulate underwater vacuum preloading of soft clay in centrifuge model tests. Two vertical drain groups were installed within one model with ideal full coverage of membrane on the soil surface. Underwater vacuum was provided to simulate the consolidation behavior of soft soil. Model preparation procedures and vacuum generation techniques are described.Response of pore water pressure, deformation and change of undrained shear strength are presented and the influence of drain spacing on the rate of consolidation is discussed.