The boiler applies the combined grate combustion system. Steam and water of the boiler system applies the natural circulation, and the downcomers are arranged outside the furnace. The boiler is “M” type, the furnace and superheater pass apply overall membrane wall, which guarantee the sealing of the boiler. The superheated steam is heated in four stages, and desuperheated by three stages of the spray water, thus the superheated steam has enough control allowance to guarantee the boiler steam parameters. There are two stages of economizers and five stages of flue gas cooler at the end of shaft. Air preheater is arranged outside the flue gas duct using the way of feeding hot water, which avoids low temperature corrosion at end of gas duct.