An optical device and its fabrication method are provided. The method includes: providing a substrate including a coupling region; forming a first dielectric layer on the substrate; forming an initial waveguide groove in the first dielectric layer on the coupling region; forming a patterned layer on a surface of the first dielectric layer and in the initial waveguide groove, exposing at least a portion of a bottom of the initial waveguide groove; and using the patterned layer as a mask to etch the first dielectric layer, to form a waveguide structure on the substrate. The waveguide structure includes a waveguide end structure on the coupling region.