This paper analyses spatial distribution of injection carriers in the active layer of planar stripe lasers with deep Zn diffusion. It is elucidated theoretically that the spreading current along the junction direction in the P-AlxGa1-x As cladding layer of the planar stripe lasers with deep Zn diffusion can be neglected. Therefore, spatial distribution of injection carriers in the active layer of the planar stripe lasers with deep Zn diffusion is much narrower than that of the conventional planar stripe lasers. Consequently, its gain-induced guide is much narrower than that of the conventional planar stripe lasers too. This proves theoretically the conclusion in [1], i. e. it is the narrowing of gain-induced guide of the planar stripe lasers with slightly doped or undoped active layer and deep Zn diffusion that causes the stabilization of the lateral modes.