For achieving a stable operation of coal gasification, it is crucial to understand the relationship between the microstructure and thermal conductivity of coal slags. Thermal conductivity measurements were carried out on a SiO2-Al2O3-CaO-Fe2O3-MgO slag system with a mass fraction of silica and alumina (SiO2 + Al2O3) ranging from 35 to 85 wt % using a nonstationary hot wire method. The crystallization tendency was obtained by a differential scanning calorimetry signal. Raman spectroscopy and X-ray diffraction (XRD) were combined to investigate the microstructure. The addition of (SiO2 + Al2O3) leads to a significant increase in the thermal conductivity of slags when (SiO2 + Al2O3) is higher than 55 wt %. The XRD results show that a gradual increase in the silica and alumina contents causes crystalline phase transition, resulting in an increase of bridging oxygen in the slag system. The degree of polymerization Q with variable (SiO2 + Al2O3) is quantified by Raman spectroscopy. The positive linear relationship between the thermal conductivity and Q is found at 1573 K owing to the high dependence of thermal conductivity on the activation energy E of crystallization tendency at this region where the crystal-melt coexists. The presence of various defects in the crystalline phase at 1173-1523 K leads to a decrease in the phonon mean free path, resulting in an exponential relationship between the thermal conductivities and Q. At this time, the thermal conductivity increases remarkably with the degree of polymerization when Q >= 1.7.