Biomass-fired boilers generally experience severe deposit problems, in particular on the intermediate temperature superheaters (SH2). In order to investigate the formation mechanisms of the deposit, mature deposits collected from the superheaters of a 30 MW biomass-fired power plant were analyzed using SEM-EDX, XRD, and XRF techniques. The results show that the deposits formed on the SH2 have an obvious layered structure, and temperature-dependent layers exist close to the tube wall, where the crystallization process is mainly affected by the steam temperature. Referenced to the three phase equilibrium diagram data in the silicate cement field, the eutectic salt KCl-K 2SO4-Na2SO4 may be formed in a secondary step after KCl and K2SO4 condense on the tube surface. The chemical subsequence processes are also capable of analyzing the deposits formation process and their composition along the thickness of the deposits of any of the three superheaters in this case. As the surface temperature of the deposits increases along the thickness of the deposits, NaCl and Na2S2O3 can be deposited together with the components KCl, K2SO4, K2Ca(SO4) 2·H2O, K3Na (SO4)2, CaSO 4, and SiO2. K2Ca(SO4) 2·H2O and K3Na(SO4) 2 are produced through the secondary reaction in all layers after they have deposited. In addition, monocrystal KCl (melting point 770 °C) was found in the top layer of the deposits. The cone shape of the layered structure of the deposits also indicates that the formation of the deposits may also be affected by the pattern of the gas flow.