A semiconductor structure and a fabrication method of the semiconductor structure are provided. The semiconductor structure includes a substrate, a gate structure over the substrate, and a sidewall spacer structure located on a sidewall surface of the gate structure. The sidewall spacer structure includes a first sidewall spacer, a second sidewall spacer, and a cavity located between the first sidewall spacer and the second sidewall spacer. The first sidewall spacer is located on the sidewall surface of the gate structure. A top surface of the cavity is above a top surface of the gate structure, and a bottom surface of the cavity is coplanar with a bottom surface of the gate structure. The semiconductor structure also includes a source and drain plug located over the substrate on each side of the gate structure. The source and drain plug is located on a sidewall surface of the second sidewall spacer.