In the conventional fiber drawing process, the waveguide parameters (fiber core diameter, refractive index profile, numerical aperture) of the optical fiber are decided (with very limited perturbation range) once the optical and geometric properties of the preform are determined. In comparison, yttrium aluminosilicate (YAS) fibers prepared by "molten core method" (also termed as "melt-in-tube" method) reveal a unique fiber formation process. Although the previous works have shown that the element diffusion plays a significant role in the formation of YAS fiber, the description of this formation process is still vague and the properties of YAS fibers obtained by this method show low controllability in practice, which cannot be explained by elemental diffusion alone. Here, a new model for the formation mechanism of YAS fibers is proposed based on the experimental data, which involves the consideration of interfacial stress during the formation of YAS fibers. The relationship between the refractive index and the element distribution in the YAS fibers was studied. Structural simulation based on the molecular dynamics was also performed to demonstrate the formation mechanism of glassy YAS fiber core.