Abstract: Slagging has long been a critical issue jeopardizing the safety and economic operation of utility boilers. The primary constituents of coal ash are aluminosilicate compounds, and the extent of silicon participation during their formation significantly influences the fusibility of the resulting products. To gain deeper insights into the formation mechanism of coal slag, this study employs molecular dynamics simulations utilizing the PCFF force field with the Garofalini potential, which incorporates both two-body and three-body interactions, to investigate a molten aluminosilicate system derived from a slagging-prone coal fly ash. We systematically analyze the evolution of key microstructural parameters—including radial distribution functions, coordination numbers, oxygen speciation, and structural units—as a function of Si content, and explore its impact on the slagging propensity. The results demonstrate that when the Si content ranges from 45% to 55% of the analytical value in the ash, the aluminosilicate melt resides in a low-viscosity zone. Within this range, the proportion of complex silicon structural units first decreases and then increases, a trend consistent with the observed viscosity changes. The minimum melt viscosity occurs at a silicon content of 50%, indicating the involvement of non-clay mineral silicon, which warrants particular attention in slagging research. When the silicon content exceeds 55%, increased Si content leads to fewer Si-Ca and Al-Ca connections. Silicon primarily bonds with oxygen to form more stable structural networks, resulting in an overall increase in system viscosity. Conversely, when the silicon content falls below 45%, the dual role of Na? and Ca2? ions—compensating charges and depolymerizing the aluminosilicate network—becomes more pronounced, causing the melt viscosity to initially increase and then decrease as Si participation is further reduced.