The peak load regulation operation of coal-fired units is an important measure to improve the consumption capacity of renewable energy, but it will have an evident impact on fuel combustion process and system. Fly ash properties are closely related to ash slagging behavior, dust removal equipment performance and particulate matter emission. However, little research has been done in the past. The influence of load change of coal-fired boiler on fly ash characteristics was studied to guide the prevention and control of ash related problems such as furnace fouling and slagging and particulate matter emission. In this work, fly ashes were sampled before ESP in a 1 000 MW coal-fired boiler under 60% and 90% operating load. Chemical composition, mineral species, particle size distributions and shape characteristics of fly ash were analyzed by advanced computer automatically controlled scanning electron microscope(CCSEM). The fly ash fusibility was analyzed by a digital imaging coal ash fusibility system. The effects of boiler operating load changing on ash properties and ash melting behaviors were investigated. The results show that changing of operation load has insignificant effect on fly ash chemical composition. Degree of mineral interaction is the key factor affecting the particle size distribution of fly ash. Low operation load of boiler suppresses the migration of Ca and Fe bearing minerals in coal to aluminosilicates, which results in finer fly ash particle size distribution. The D50 of fly ash decreases from about 40 μm to 30 μm. Compared withfly ash in low load operation condition, the melting of fly ash in the furnace under the low load operation condition is restrained. The proportion of molten fly ash from low load operation condition is about 10% lower than that from the high load operation condition, which is mainly attributed to the lower combustion temperature under low load operation condition.