The fusion characteristics of ash mixtures of two lignites (Husheng and Xiaolongtan) and three biomasses (peanut hull, corn straw, and pine sawdust) were investigated by measurements using ash fusion temperature (AFT) detector, X-ray diffraction, normalized reference intensity ratio, analysis using Rietveld-based SIROQUANT software, and scanning electron microscopy. AFT modification of lignite mixed ashes is not the same as the addition of different biomasses at the same mass ratio, which depends upon their ash compositions, especially on the mineral arising from mineral evolution with the temperature increase. The formation of high-melting-point (MP) mullite and change in its content might be the main reason for AFT fluctuation of the lignite mixed ashes. With the increase in mass ratio of biomass, the amount of low-MP minerals and their eutectic increases at a high temperature, leading to the decrease in AFT of the mixed ashes.