A method for arch single-stage injection fractional combustion and a W flame boiler for realizing the method relate to a fractional combustion method and a W flame boiler. The invention aims at the problems that the W flame boiler has high NOx discharge, high flying ash combustible content, serious lagging on water cool wall front and behind a lower hearth and inferior flame stability. The method of the invention comprises the steps of gradually injecting through arch secondary air with an air speed of 35 to 65 m/s, taking thick coal powder airflow with the air speed of 10 to 20 m/s to inject downwards, and injecting the arch secondary air with the air speed of 35 to 65 m/s into the furnace through an arch secondary air nozzle. The boiler of the invention comprises a thick coal powder airflow nozzle, an arch secondary air nozzle and a thin coal powder airflow nozzle which are arranged on the furnace arch from the center of the furnace to the front and back water cool walls, wherein the thick coal powder airflow nozzle is abutted to the arch secondary air nozzle and an arch lower secondary air nozzle is arranged on the front and back water cool walls on the lower furnace along the width direction of the furnace. The W flame boiler for realizing the method of the invention can decrease the W flame boiler and the flying ash combustible content.