A 75 t/h biomass circulating fluidized bed (CFB) boiler burns mixed fuels for long time，including eucalyptus bark，wood tail，wood board，bamboo，and industrial composite board. On the high-temperature superheater and low-temperature superheater，a severe slagging phenomenon was observed during actual operation. As a result，it's critical to investigate the slagging mechanism of mixed biomass fuels while considering the characteristics of various biomass fuels. The slagging samples from a biomass CFB boiler's high-temperature superheater and low-temperature superheater were collected firstly. Following that，the physical and chemical properties of the collected samples were investigated by analyzing the ash composition and ash fusion temperatures. The results show that among the biomass used in the boiler，bamboo，wood tail and wood board have high calcium and potassium content，with approximately 30% CaO content and more than 10% K2O content. Eucalyptus bark contains high silicon and aluminum，with an ash content of SiO2 as high as 62.4%. Industrial composite board and wood tails have high levels of chlorine，and the chlorine content in the ash is 0.67% and 0.865%，respectively. In addition to bamboo and Industrial composite board，others have high ash fusion temperatures with a soften temperature above 1 200 ℃. The slagging samples from high-temperature superheater area have a layered structure：the outer layer of the slagging sample is hard，the middle layer contains white KCl crystals and fine particles，and the outer layer of slagging and ash particles are made of Ca2Al2SiO7，SiO2，KCl，Ca2MgSi2O7. The low-temperature superheater's outer slagging samples have the same composition as the high-temperature superheater's. However，white KCl crystal content in the middle layer is higher，which is related to the condensation and precipitation of KCl vapor in the flue gas.