A double-sided twill stereo glass fiber fabric and a method for preparing a composite material are provided. The glass fiber fabric has a 2/6 double-sided twill imitated stereo structure, has the surface density of 600+/-30 g/m<2> and the thickness of 0.70+/-0.05 mm, has excellent flexibility, surface smoothness and mechanical properties, and is especially suitably used as a reinforcing material of a resin-based composite material. Preparation of the composite material comprises the steps: firstly, continuously impregnating a fabric with a resin solution in virtue of a solution-method impregnator to prepare a prepreg; and then, making the prepreg into a composite material having a laminated structure or a composite material having a sandwich-layer structure including a honeycomb sandwich-layer structure and a foam sandwich-layer structure according to design requirements. Various mechanical properties of the prepared composite material are excellent in expression, and especially, the single-layer thickness of the prepreg after curing in the composite material reaches 0.55-0.60 mm and is significantly higher than that of prepreg of glass fiber fabrics with satin weave, plain weave, twill and other types and with the same unit weight; therefore, the glass fiber fabric can be widely promoted and applied in cabin internal decoration of high-speed rails, ships and the like.