Abstract: Based on the theory of HPM propagating in mixed-atmosphere, studies are conducted on the phenomenon and principle of self-focusing/defocusing of the narrow HPM beam propagating in ionospheric plasma. The nonlinear propagating equation, and focusing/defocusing parameters have been derived with nonlinear heating effects being considered. The dependence of beam width on the frequency and intensity of HPM has been illustrated in the presence of magnetic field. Simulations show that the lower ionosphere mainly represents a defocusing medium. The divergent phenomenon occurs for L-wave, R-wave, O-wave and X-wave. And the angle of divergence is the maximum for R-wave and the minimum for L-wave. But the difference will decrease with the increase of the frequency of incident wave. Defocusing effect