The properties of coal slags in slagging gasifiers such as viscosity are intrinsically determined by the structure. In this article, the structure of 10 coal slags is investigated by a combination of the nuclear magnetic resonance (NMR) experiment and molecular dynamics (MD) simulation quantitatively. The proportions of different structural units Q(n) with Si as the centered atom are determined, and the results from the NMR experiment and MD simulation agree well. As the basicity increases, the proportion of the Q(4) unit decreases, while those of Q(3), Q(2), and Q(1) increase, which indicate that the polymerization degree decreases with an increase of basicity. A structural parameter, Q(Si), is proposed to characterize the polymerization degree, and the relationship between Q(Si), and basicity can be described by a linear equation. Also, an exponential function between the viscosity and the proposed polymerization parameter Q(Si), is discovered. Viscosity experiments are performed to verify the predictions, and the result shows a good consistency between experiments and predictions above the temperature of critical viscosity (T-cv).