It has been found experimentally that the planar stripe lasers with deeper Zn diffusion have more stable horizontal transverse mode. A simple equivalent circuit model is proposed that can explain why the planar stripe lasers with deep Zn diffusion have stabilized horizontal transverse modes. It is shown analytically that the narrowing of gain-induced guide of planar stripe lasers .with undoped active layer and deep Zn diffusion is responsible for the stabilization of the horizontal transverse modes.