The invention discloses a method of improving product stability of segmented common bar screw-thread steel. The method comprises the following steps: obtaining lateral temperatures and gradient distribution conditions of parallel rolled pieces according to a preset bar segmenting and rolling process database, giving executing parameters of a lateral temperature heat supplementing device of the parallel rolled pieces, and supplementing heat by the lateral temperature heat supplementing device of the parallel rolled pieces in light of the parameters; detecting temperature distribution of the segmented parallel rolled pieces by using a temperature detector, giving executing parameters of one flame combustion nozzle in N segmenting number among torn parallel rolled pieces behind a segmenting wheel, and supplementing heat by the flame combustion nozzle in light of the parameters; meanwhile, giving water flow and water pressure parameter values of water tanks of cooling water tank groups corresponding to lines by means of detection values of the temperature detectors and cooling the rolled pieces of the lines by the cooling water tank group; and meanwhile, calculating rolling reduction of a finisher by means of detection values of the temperature detectors and rolling the rolled pieces of the lines by a finisher set.