The paper briefs the major features of main part and system design of circulating fluidized bed (CEB)boiler;emphatically analyzes the important contents and existing problems in the commissioning debugging process of larze size circulating fluidized bed boiler;in view of the debugging praetice of HG-480/137-L.MG21 type CFB boiler (burning Yangquan anthracite),puts forward the following debugging optimization scheme of:air quantity calibration,high temperature refractory curing,steam blow-cleaning,furnace heating surface protection,going into operation time and mode of loop seal system, going into operation of main fuel,coalfeeding control and under-bed igniting burner etc.