The difficult synthesis and low purity of 413 high-entropy MAX phase powders have severely constrained their development in the field of electromagnetic wave absorption. To address this, the present study optimized a two-step solid-phase synthesis process. By examining how different raw-material pretreatment methods affect the structural evolution of carbide precursors, we proposed a “structural genetic mechanism” that systematically explains the synthesis pathway of the 413 high-entropy MAX phase. The results show that cubic-phase carbide precursors are essential for producing 413 high-purity high-entropy MAX. Using the optimized method, we synthesized pure-phase 413 high-entropy MAX and, for the first time, successfully prepared (Mo_0.2Ta_0.2Nb_0.2Ti_0.2V_0.2)₄AlC₃ via a conventional pressureless solid-state reaction route. Wave absorption tests indicate that the synthesized 413 high-entropy MAX phases all exhibit good absorption performance. In particular, (Mo_0.2Cr_0.2Nb_0.2Ti_0.2V_0.2)₄AlC₃ achieved a maximum effective absorption bandwidth (EAB_max) of 4.24 GHz at a thickness of 1.57 mm and a minimum reflection loss (RL_min) of −52.88 dB at 1.87 mm. (Mo_0.2Ta_0.2Nb_0.2Ti_0.2V_0.2)₄AlC₃ showed a clear advantage at small thicknesses, with an EAB_max of 3.44 GHz at 0.93 mm and an RL_min of −51.60 dB at 0.89 mm. This study fully demonstrated the effective regulation of wave absorption performance by high-entropy engineering and provided an effective approach to expand the types of 413 high-entropy MAX phase powders, offering a useful reference for exploring this material in the field of wave absorption.