The present disclosure provides a semiconductor device and a fabrication method. The method includes: providing a substrate and forming initial fins on the substrate. The initial fins include a sacrificial material layer and a first material layer on the sacrificial material layer, first trenches are formed between adjacent initial fins, and the first trenches expose the substrate. A first layer is formed in the first trenches. Second trenches are formed in the initial fins. The second trenches expose the substrate, the sacrificial material layer is formed into a sacrificial fin layer, the first material layer is formed into fins, and the fins are located on the sacrificial fin layer. The sacrificial fin layer is removed to form first fin openings between the substrate and the fins. An isolation structure is formed on the substrate and in the first fin openings.