To address the serious slagging issue in biomass boilers, field sampling, compositional analysis and corresponding FactSage thermodynamic calculations were conducted to study the slagging characteristics of a straw/bark-fired grate boiler. In addition to clarifying the multiple mechanisms, including the initial deposition by gaseous condensation and the fly ash capture by the viscous initial layer, the contribution weights of various mechanisms to the slagging behavior in four superheater areas were also obtained. Results indicate that K/Na double sulfates, rather than regular alkali sulfates, play a key role in forming the initial layer by condensation in high-temperature superheater. In 2nd mid-temperature superheater, the condensed KCl and less sulfate form high-viscosity initial layer, which captures fly ash easily and results in serious slagging. The composition evolution during slagging process simulated by FactSage indicates that low-temperature eutectics formed at high temperature area are more easily captured by the initial layer of high temperature superheater. The sulfates of K and Na start to combine below 950ºC and completely form K3Na(SO4)2 at 750ºC. K-Ca double sulfates exist above 850ºC, while below 750ºC completely decompose into K2SO4 and CaSO4. The gaseous condensation deposition contributes over 45% in the slagging of high/mid-temperature superheater, and only 8.4% in low-temperature superheater. The results provide a basis for the slagging prediction and prevention in biomass-fired grate boiler.