The invention discloses a composite LCP high-frequency high-speed double-sided copper foil substrate, which includes at least one LCP core layer, at least one extremely low dielectric adhesive layer, and two copper foil layers. The LCP core layer and the extremely low dielectric adhesive layer are located between the two copper foil layers. The invention has not only good electrical performance but also low roughness, stable dk/df performance in high temperature and humid environment, ultra-low water absorption, good UV laser drilling ability, low spring force suitable for high density assembly, and excellent mechanical properties. Besides, in current technology, only a thickness limited to a maximum of about 50 μm can be obtained in the coating method. In the preparation method of the invention, a substrate having a thickness of 100 μm or more can be easily obtained.