<p>The current research on the manufacturing of large-scale and complex components focuses mainly on the casting processes. Compared with casting, plastic forming has significant advantages in terms of performance. However, effectively controlling the material flow to achieve a reduced loading force and near-uniformity in the isothermal plastic forming process of large-scale asymmetric magnesium alloy complex housings (LSMACHs) is challenging. This study proposes a material flow control method based on the diffluence upsetting-extrusion forming (DUEF) process by dividing different forging deformation regions, combining these with the principal stress method, and establishing an efficient and accurate design procedure. A rational preformed billet was designed successfully using this method. Subsequently, a finite element simulation was employed to analyze the multiphysics fields of the DUEF process. The results indicated that compared with the traditional closed-die forging (TCDF) process, the DUEF process could control the orderly flow of materials, achieve short-distance filling of materials, and reduce hydrostatic stress. Simultaneously, it improved the deformation uniformity by 20.3% and reduced the loading force by 22.6%. Finally, the rationality of the proposed method was validated through physical experiments. Compared with the TCDF process, the DUEF process exhibited a low loading force and uniform mechanical properties. The proposed material flow control method based on the DUEF process provides a new technological approach for the plastic formation of LSMACH and similar components.</p>