The disclosure provides an M-type pulverized coal boiler suitable for ultrahigh steam temperature. The pulverized coal boiler comprises a hearth of which the bottom is provided with a slag hole and a tail downward flue of which the lower part is provided with a flue gas outlet. The pulverized coal boiler further comprises a middle flue communicated between the hearth and the tail downward flue, wherein the middle flue comprises an upward flue and a hearth outlet downward flue of which the bottoms are mutually communicated and the upper ends are respectively communicated with the upper end of the hearth and the upper end of the tail downward flue to form a U-shaped circulation channel. In the pulverized coal boiler provided by the disclosure, the middle flue which extends downwards and can make flue gas flow along the U-shaped circulation channel is arranged between the outlet of the hearth and the tail downward flue, so that high-temperature flue gas from the hearth can be introduced into a position with low elevation through the downward flue, and final-stage convection heating surfaces (such as a high-temperature superheater and a high-temperature reheater) can be arranged at positions with low height, and the length of ultrahigh-temperature steam pipelines between the high-temperature superheater and a steam turbine, and between the high-temperature reheater and the steam turbine can be greatly reduced. Therefore, the manufacturing cost of a boiler unit is obviously reduced.