A model of sheath formation on a planar electrode in strongly electronegative plasmas is presented.The plasma sheath is studied by using a set of hydrodynamic equations.Spatial potential,net space charge distributions and sheath width as functions of distance are obtained.It shows that in a strongly electronegative sheath a presheath between bulk plasma and sheath hardly exist,and electrons,negative and positive ions in the sheath form a pure positive ion sheath near boundary of electrode.Density of net space charge has a sharp peak near the sheath edge.Compared with an electropositive sheath,it is found that the width of strongly electronegative sheath is much narrower,and the spatial potential within the sheath falls much faster.