Boiler blowdown affects not only the unit replenishment rate,but also the thermal economy of the unit operation.The operation characteristic of a 600 MW unit boiler was analyzed to investigate the influence of different drainage modes on thermal efficiency of the unit.The results showed that,continuous blowdown was superior to periodical blowdown,especially for the boiler system with only steam expanding vessels.The blowdown flow rate needs to be determined by the boiler water.