The present disclosure provides a composition for preparing a lightweight transparent composite, a method of preparing a composite using the same, and a composite prepared by the method. The lightweight transparent composite having excellent impact resistance, scratch resistance, weather resistance, and high hardness can be prepared by thermally curing a mixture of a glass fiber reinforcement having a refractive index of 1.4 to 1.6 and a curable resin having a deviation in refractive index of −0.005 to +0.005 from the glass fiber reinforcement. When the lightweight transparent composite is applied to transparent windows for automobiles, the lightweight transparent composite can be useful in enhancing fuel efficiency, achieving easier handling characteristics due to reduction in vehicle center of gravity, and improving juddering and dimensional stability even during car driving due to excellent hardness.