The invention belongs to the technical field of composite molding technologies, and particularly discloses a method for molding an ultra-light plane control surface through a composite liquid. The method comprises the steps of manufacturing parts like upper and lower edge strips and webs of a beam; carrying out vacuumizing and curing molding; putting the upper and lower edge strips and the webs on an aluminium plate to be laid, and forming a beam structure; carrying out secondary vacuumizing, introducing resin, and carrying out curing and demolding to obtain the beam structure; and manufacturing upper and lower skin, assembling the upper and lower skin with the beam, supporting hollow parts of the upper and lower skin through fine sand or air inflation during assembly, and after bonding and curing molding, discharging fine sand or air. The invention further provides the ultra-light plane control surface based on the technological method. The method has the advantages that glue injection is carried out on the webs and the lower edge strip through resin infiltration, and the vacuum introduction problem of complex three-dimension shapes is solved; and bags wrapping fine sand are adopted as a filler, so that bonding quality of the skin under high temperature and high pressure is guaranteed, the problem of taking out the filler in the skin is solved, and residues are avoided.