Hall thrusters with large height-radius ratio, owing to their unique advantages in compactness, lightweight, and high performance, have progressively emerged as a preferred choice for diverse space propulsion applications in the future. However, the amplification of the annular effect in structures with a large height-radius ratio poses a practical problem of plume over-focusing, which seriously restricts the further improvement of Hall thruster performance and the extension of its life. In this study, the formation mechanism of over-focused plume is deeply investigated, and it is ascertained that an intensified radial electric field directed towards the inner wall within the channel serves as a key contributing factor. This phenomenon is fundamentally attributed to structural characteristics of large height-radius ratio that induce pronounced inward inclination of field lines within strong magnetic field zone. Based on this, the design concept of focused magnetic field is proposed, wherein straight magnetic field lines are established within the strong magnetic field zone to generate a quasi-axial accelerating electric field. Simultaneously, the symmetrical magnetic field inside the channel ensures ionization concentration near the channel center, thereby achieving optimal matching between the ionization zone and accelerating field. Experimental results demonstrate that employing a focused magnetic field significantly reduces the divergence half-angle of the plume and yields an excellently barrel-shaped focusing plume morphology in HEP-1350PM. Consequently, the total efficiency of the thruster surpasses 60%, while erosion belt on the inner wall is shortened by nearly 50%. These advancements effectively enhance thruster performance and prolong its operational lifespan. This study can not only resolve practical problems associated with plume over-focusing, but also provide a fundamental guiding principle for magnetic field design of Hall thrusters.