The invention discloses a method for improving impact absorption energy of a flange in an ultralow-temperature state. The flange is connected on a large wind power tower cylinder. The method comprises the following steps of: selecting Q345 steel as a material for the flange, and controlling the content of alloy elements and the weight percentage content of fine grain trace elements in the steel; simultaneously setting a heat treatment process for the flange, and according to the nominal thickness and the environmental temperature of the flange, performing an normalizing process or performing a blowing cooling process after normalizing and discharging or performing an intermittent water cooling and tempering process after normalizing and discharging; and setting the mechanical property sampling part of the flange at about 20 millimeters in the inner ring of the flange towards the outside diameter, and setting the sampling direction in the tangential direction or the radial direction orthe axial direction. The impact absorption energy performance of the flange produced by using the method in the ultralow-temperature (50 DEG C below zero) state is remarkably improved by the improvedheat treatment process.