Abstract: Lake sediment, as a solid waste, can be disposed harmlessly through combustion. The ash melting temperature is an important factor affecting boiler coking and restricting its safe and stable operation. Determining the ash melting temperature after mixing lake sediment with coal is of great significance for achieving the resource utilization of lake sediment. This paper takes the Yunnan Xiaolongtan lignite and the Dianchi Lake sediment as the research objects. The Dianchi Lake sediment is a high moisture solid waste with high nitrogen content and rich heavy metal elements. The Yunnan Dianchi Lake sediment has a large residual stock, mainly generated by a large amount of sediment entering the Dianchi Lake with river water and rainwater. It is the main source of pollution in the Dianchi Lake. However, the Xiaolongtan lignite has problems such as high production, poor coal quality, and high transportation costs, which seriously affect its utilization rate, if the local conditions can be adapted and the problems of high production, difficult treatment, and insufficient efficiency of local sediment can be solved through mixed combustion, in addition to achieving an efficient utilization of lignite, it can also alleviate the local disposal pressure. However, there is currently little research on the blending of lignite with sediment. Through experiments on the ash melting characteristics, the deformation, softening, hemisphere, and flow temperature of ash at a 5%−15% mixing ratio were determined, and the variation law of ash melting point was derived. The mineral composition, oxide, and elemental composition were obtained through XRD, XRF, and SEM-EDS detection and analysis. The crystal structure and mineral composition in the ash were confirmed, and a ternary phase diagram was constructed for thermochemical calculations. The results show that when 5% sediment is mixed, the melting temperature of the ash decreases sharply, and then the downward trend tends to be gentle. The acid-base ratio and silicon aluminum ratio are positively correlated with the downward trend. The oxides of Fe⁺², Fe⁺³, and Ca⁺² play a main role in reducing the melting point of the ash, and these oxides are prone to react with refractory minerals to form fluxing minerals such as anorthite and calcium iron pyroxene. During the low temperature stage, Ca oxides mainly promote melting, while during the high temperature stage, Fe oxides act.