Abstract: Zhundong Coal field has a high predicted coal reserve.The ash generated by Zhundong coal is characterized by high content of Sulphur,alkali/alkaline，and low content of Si and Al.Sever fouling,slagging problems occurred in Zhundong coalfired boilers,which greatly obstruct the largescale utilization of Zhundong coal.The formation/decomposition of Na2SO4 at high temperature may be affected by SO2 and SO3 in flue gas.Thereby affecting the melting process of coal ash.In-depth study on the influence of the flue gas atmosphere on the melting characteristics of coal ash will help deepen the understanding of the slag process.Which can provide technical support for the slagging prevention in real boilers.To acquire the influence of flue gas atmosphere on the melting characteristics of Zhundong coal ash,a high temperature online microscopic observation test rig based on single hot thermo-couple technique (SHTT) was constructed.The melting characteristics of Zhundong ash under the reducing,oxidizing and inert atmosphere were compared.The results show that 96.92% of the measured ash melting point through SHTT test rig is consistent with the standard melting point measurement method,and within an uncertainty of 3%.When alkali/acid ratio R is lower than 2.5,the atmosphere have little effect on ash melting point.When R is larger than 2.5,the high Fe2O3 content causes a lower ash melting point at the reducing atmosphere.In real flue gas atmosphere,the melting point of Zhundong ash increases since the existence of SO2 restrains decomposition of CaSO4 in Zhundong ash as well as the production of low melting materials such as feldspar and augite.SO2 in flue gas also stimulates the production of ultra-fine particles rich of Na/Fe sulfates and thus plays a significant role on fouling and deposition.