A method for producing ultralow-carbon steel based on a straight tube vacuum refining device, provided by the invention, is characterized in that the ultralow carbon steel is produced through an inward side blowing hole combination uniformly circumferentially arranged along the lower edge of an impregnating tube wall and independently controlling the flow in 2N segments (N = 3 to 6) during ladle-free bottom blowing. An asymmetric gas blowing mode of blowing of a large amount of argon to the inward side blowing holes arranged in the semicircumference of the lower edge of the impregnating tube wall and blowing of a small amount of argon to the side blowing holes arranged in the semicircumference at the opposite side during depth decarburization is adopted; and blowing of a large amount of argon to all the inward side blowing holes arranged along the lower edge of the impregnating tube wall also can be adopted to push top slag covering the surface of molten steel in a vacuum chamber to amiddle area and enlarge the bare area of the surface of the molten steel in the vacuum chamber. Above two cyclic decarburization modes can utilize a vacuum carbon and oxygen reaction on the surface ofthe vacuum chamber to achieve decarburization, and also can utilize iron oxide in the oxidizing top slag in the vacuum chamber to accelerate the surface decarburization rate of the molten steel.