This paper presents the measurement of effective heat conductivity of ash deposit based on a novel technique for Zhun Dong coal (ZD) with high sodium in a pilot scale furnace. The variation of deposit thickness with time is determined by digital image technique. In addition, simplified two color method is applied to obtain the deposit surface temperature. Subsequently, the change of the average effective heat conductivity of the ash deposit with deposit growth is calculated by combining the deposit surface temperature and thickness. Meanwhile, mineral phases of the ash deposits were identified by X-ray diffraction spectrometry (XRD); scanning electron microscopy (SEM) was applied to analyze the microstructure of ash deposit, and the corresponding chemical compositions were determined by energy dispersive X-ray spectrometry (EDX). The growth process of the ash deposit on probe 2 was composed of four stages based on digital image technique. This was confirmed by the layer structures of the slagging deposits. In addition, the deposit surface temperature increased with time, and it finally fluctuated around 1255 degrees C. Meanwhile, the results showed that the average effective heat conductivity of the ash deposit on probe 2 increased linearly with deposit growth. The XRD analysis results of three deposits showed that these deposits had similar mineralogy in every layer structure. The content of gehlenite in the deposit increased with the deposit growth. Moreover, the loss of glass is accompanied by a corresponding increase in crystalline phase.