Abstract: To discuss the mechanism of sawdust slagging, slag was formed by controlling gasification conditions on a self-made updraft biomass gasifier, the slagging condition and location were analyzed, and slag samples were also characterized in detail. The results showed that, as air flow rate increasing, the oxidation temperature of the gasifier increased sharply, and the gasification intensity enhanced obviously. As air flow rate exceeded to 3.5 m3/h, the gasification intensity was higher than 112.7 kg/(m2·h), and the oxidation temperature was up to 1?129℃, thus slagging was easy to generate inside the gasifier. The slagging in the updraft gasifier belongs to over-temperature slagging, and it was mainly formed in the oxidation zone at the bottom of the gasifier by the biomass ash melted and bonded, and finally condensed in the cooling process after shutdown the gasifier. The slag sample appeared block shape and gray-black color, with very low carbon content, hard texture and compact structure. High-temperature melting and cooling condensation caused its internal pore structure collapse and the surface pore occlusion, the specific surface area and pore volume were both very low. During the high temperature slagging process, the content of the low melting point and highly volatile elements such as alkali metal (K), alkaline earth metal (Mg, Ca), P, S, etc. decreased, while the high melting point and difficult volatile elements such as Al, Si, etc. were enriched. Compare with the biomass ash prepared at 550 and 815℃, high temperature melting changed the crystal form, and besides SiO2, much silicate eutectic was also formed in the slag samples including Ca(Mg, Al, Fe)Si2O6, CaMg(SiO3)2, KAlSi3O8, K(Si3Al)O8 and Ca(Fe, Mg)(SiO3)2, etc.