A semiconductor structure and a fabrication method are provided. The semiconductor structure includes: a base substrate, an optical waveguide layer over the base substrate; a first dielectric layer over the base substrate; a cavity between the first dielectric layer and the optical waveguide layer; and a second dielectric layer on the first dielectric layer and the optical waveguide layer. The cavity is located on sidewall surfaces of the optical waveguide layer and has a bottom coplanar with a bottom of the optical waveguide layer. The second dielectric layer is located on a top of the cavity and seals the cavity.