In an entrained flow gasifier, the viscosity of molten slag along the wall is critical for continuous operation. Molten coal slag belongs to silicate-containing melts and slag properties are intrinsically affected by structure transformation. In this article, the structure of coal slags with different SiO2 compositions ranging from 48% to 63% wt is investigated by solid-state nuclear magnetic resonance (SS-NMR) spectroscopy and molecular dynamics (MD) simulation. Si-29, Al-27 spectra were obtained and the contents of Q(n) species were analyzed. Radial distribution functions, proportions of bridging oxygen (O-b) and non-bridging oxygen (O-nb) were calculated. Viscosities of slags were measured by a high temperature viscometer. Both NMR measurement and MD simulation show that an increase of silicon can lead to a high degree of polymerization. Most alumina is in form of the four-coordinate structure like the tetrahedral Si. The chemical bond of Si-O is the most stable compared to other bonds such as Al-O, Ca-O etc. Therefore, an increase of silicon can promote the [SiO4] formation and form more O-b while lowering the amount of O-nb. The increased polymerization degree with higher silicon leads to a high slag viscosity and a linear relationship is found between the measured viscosities and fractions of Q(4).