Tremendous sodium-rich coal in China has unique ash compositions, and the entrained flow gasification is one of the best choices to use sodium-rich coal for coal chemical industry. The entrained flow gasifiers require smooth slag tapping for long term and safe running, but the viscosity-temperature behavior of high-sodium slags is unknown, which limits the utilization of sodium-rich coal. In this study, viscosity-temperature behavior of high sodium coal ash slags was revealed for the first time, and the parameter of slag network structure and index of slag crystallization tendency is raised for slag viscosity evaluation. The results show that Na2O provides O2- ions which break Si-O-Si bonds and slag network structure. However, Al3+ ions are absorbed into silicate network, acting as a network former with the ionic charge-compensation effect of Na+. The classic structural parameter (fraction of non-bridging oxygen, NBO) is modified to evaluate the high-sodium slag viscosity accurately by taking Al3+ into account. NBO fraction decreases as Na2O content increases, leading to the decrease of slag viscosity. Below T-liq, slags are prone to be crystalline slag with increasing Na2O content or glassy slag with increase in SiO2/Al2O3 ratio (S/A). A novel index, namely glassy slag formation ability (G), is established to quantitatively evaluate the crystallization tendency of coal ash slags. G is the ratio of activation energy for viscous flow (E-eta) to T-liq. The slag will exhibit the glassy behavior when G is higher than 0.16 kJ/(mol.K). The results enhance the knowledge of viscosity-temperature behavior of high-sodium coal ash slags and will be helpful for coal selection and blending to avoid slag blockage in entrained flow gasification.