Abstract: Several biomass-fired boilers in the some region of northern China have to be shut down within average 20 30 days for the serious problem of deposition on the secondary super-heaters.In this paper,the main components of deposit samples were analyzed,which were collected on the secondary super-heaters in one of the boilers of this region,and the formation process of deposits was discussed.It showed that the high content of K,Na play the key role on the deterioration of deposition on the secondary super-heaters,while high-Ca and low Si,Al,S are also an important cause for it.Adding kaolin（the main component is Al2O3×2SiO2×2H2O） to the fuel,or co-firing with coal which has high aluminum can ease the worse deposition problem.