The boiler applies the combined grate combustion system. Steam and water of the boiler system applied the natural circulation, and 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. Three stages of economizers, primary and secondary of air preheater are aggranged in the back of superheater. The boiler can be ignited by handy firewood. The interface of the strarup burner is prepared in advance on the side furnace wall to guarantee to improve the reliability of the ignition. The boiler is of outdoor arrangement, the operation level is EL8.0m, the structure over the operation level is steel and the boiler designed as per 7 degrees of earthquake intensity.