Abstract: Jinjitan coal and sand were respectively chosen as raw material and additive. Under the different proportion of additive, ash fusion and viscosity behavior of coal ash with high contents of Fe and Ca were studied. X-ray diffraction (XRD), high-temperature stage microscope (HTSM) and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) were applied to investigate effect of mineral transformation on ash fusion and viscosity behavior of coal ash. The results show that ash fusion temperature decreases at first and then increases with the rising content of additive, and formation of low temperature eutectic augite is the main reason for the lower ash fusion temperature. The viscosity fluctuation is related to the formation of gehlenite, and the precipitation of iron-bearing minerals causes significant increase of viscosity. The temperature of critical viscosity (t_cv) of coal ash slag drops dramatically and the slag type transforms from crystal slag to glass slag with addition of sand. Distribution of Fe and Ca shows obvious different in raw coal slag, but additive makes it become more uniform, which is in agreement with the viscosity curve. Moreover, the industrial results prove that sand is an effective additive to improve the viscosity behavior of coal ash with high content of Fe and Ca.