Since a stable platform for construction of improvement of deep soft strata is the principal aim of shallow improvement of foundation with dredged slurry by vacuum preloading,slide in front of cushion advance,bearing capacity failure,punching failure should be avoided during construction of improvement of deep soft strata.Equations of the undrained shear strength of shallow improvement of foundation with dredged slurry are derived accounting for the bearing capacity failure and punching failure during improvement of deep soft strata.The undrained shear strength of shallow improvement of foundation with dredged slurry required by the stability of cushion advance should be gotten through overall stability analysis.It is shown by analysis that the undrained shear strength of shallow improvement of foundation with dredged slurry accounting for the bearing capacity failure and punching failure relates to the thickness of cushion,construction equipments for treatment of deep soft strata and thickness of shallow improvement,etc.The undrained shear strength of shallow improvement of foundation with dredged slurry required by overall stability of cushion advance increases sharply along with the increase of thickness of cushion when the cushion thickness is more than 1 m.The underlying slurry is thicker,and the undrained shear strength of shallow improvement required by overall stability of cushion advance is greater.