The invention relates to a pneumatic ash conveying method and device for controlling the material level for a thermal power plant, and belongs to the technical field of energy conservation of the thermal power plant. An ultrasonic level gauge or a radar level gauge is mounted on an electric precipitation ash hopper; a material level alarming signal is set according to the ash storage amount in the ash hopper; when passing through an electric field of electric precipitation, ash in flue gas is adsorbed to an anode plate by ionization, an anode vibrates and knocks the anode plate, the ash on the anode plate drops in the ash hopper, and the ash storage amount in the ash hopper is measured through the level gauge of the ash hopper; during high I value alarming on the material level of the ash hopper, an ash conveying unit of the ash hopper begins to convey ash; and after a high I value alarming signal disappears, ash conveying is finished. Pneumatic ash conveying can be carried out according to the ash storage amount of the ash hopper, the circumstance of ash conveying of empty tubes of a pneumatic ash conveying system is stopped, waste of compressed air is reduced, loading working time of an ash conveying air compressor is shortened, and power consumption on ash conveying is reduced effectively.