Because of the existence of initial hydraulic gradient in clay, nonDarcy equation with initial hydraulic gradient was modified with classical consolidation theories of sanddrained ground.And the approximate solution in the instance of vacuum preloading was presented.Based on this, the effect of initial hydraulic gradient on the seepage front, the pore water pressure and the average degree of consolidation were investigated.It was found that the spread of vacuum pressure was delayed because of initial hydraulic gradient, resulting in the dissipation of pore water pressure and the speed of soil layer consolidation.The hysteresis became more and more evident with the increase of initial hydraulic gradient.In addition, initial hydraulic gradient also caused some residual pore water pressure which could not reach vacuum pressure.Thus, the ultimate average degree of consolidation was less than 100%, which decreased with the increase of initial hydraulic gradient.