Abstract: A simple and practical heat flux probe was designed according to one dimension steady state heat conduction law. Simulation on ash deposition process was made with heat flux probe when four representative fuels, including Shenmu coal, Huangling coal, Xinwen CWS and black liquor coal slurry, were burned in 0.25MW test furnace. Heat flux variation and ash deposition characteristic was investigated for the fuels. The results indicate that ash probe can be applied well for simulating formation of ash deposition of different fuels. The simulation results are corresponding with the practical case. Ash depositing rate and heat flux decay rate depend on fuel characteristic mainly, meanwhile influenced by flue temperature. By analyzing apparent physical characteristic, microcosmic structure, elements composition and mineral phase of ash in probe, the result reveals ash deposition mechanism of four typical fuels is different. The content of Na, K and S in black liquor coal slurry ash is higher than else, and its dominant phase is nepheline and thenardite, with high Na and low fusing temperature. While Huangling coal ash deposited in probe is rich in Fe, Ca. Enrichment and deposition of Fe is the principal factor in ash forming process when Xinwen CWS is burned. Among the fuels tested, the slagging tendency of black liquor coal slurry and Huangling coal is stronger than that of Shenmu coal and CWS.