A general stability condition against the horizontal and vertical displacements of a plasma column in tokamak and the change of its minor radius has been deduced, while the effect of the toroidal field and the interaction of the plasma column with the external circuits are taken into account, assuming circular minor crosssection of the plasma column and the ratio α/R<<1. In general, because of the effect of the strong field, the minor radius of the plasma column would change with the change of its major radius to maintain the conservation of the toroidal magnetic flux in the angular plasma current circuit. When the variations of the currents in external circuits are considered, the stability region for such displacement pertur-bance is wider than 0