Abstract: The content of Si₈Al₆Fe₄Ca in metallurgical-grade silicon (MG-Si) influences the yield of (CH₃)₂SiCl₂. However, due to the limitations of the refining and solidification processes of MG-Si, enhancing the endogenous Si₈Al₆Fe₄Ca content is challenging. In this study, a method for synthesizing Si₈Al₆Fe₄Ca was developed by designing FeSi₂–Al–Si₂Al₂Ca multiphase reaction diffusion couple experiments. Additionally, the mechanism underlying the formation of Si₈Al₆Fe₄Ca: 4FeSi₂ + Si₂Al₂Ca + 4Al = Si₈Al₆Fe₄Ca + 2Si (1173 K) was elucidated. The results showed that the addition of an appropriate amount of Al to the FeSi₂–Si₂Al₂Ca reaction system with a mass ratio of 1:1 increased the yield of Si₈Al₆Fe₄Ca to 44.9wt%. However, the Si₂Al₃Fe generated from the direct reaction of FeSi₂ and Al contaminated the synthetic product. Incorporating Al in the form of Si(Al) solid solution within the Si₂Al₂Ca phase reduced the excessive generation of Si₂Al₃Fe from FeSi₂ and Al. The optimal component ratio of Si₂Al₂Ca was Si : Al : Ca = 27.45wt%:52.95wt%:19.60wt%, which reduced the conversion rate of FeSi₂ to Si₂Al₃Fe from 55.06% to 43.8%. Furthermore, the optimal mass ratio of FeSi₂ to Si₂Al₂Ca was 1:0.45, which increased the content of Si₈Al₆Fe₄Ca in the synthetic product to 65.74wt%. This study presents an economical and convenient method to increase the content of Si₈Al₆Fe₄Ca in MG-Si, laying a theoretical foundation for improving the efficiency of synthesizing (CH₃)₂SiCl₂ from MG-Si.