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 = 27.45:52.95:19.6wt%, 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.