A semiconductor structure and a fabrication method are provided. A fabrication method includes providing a plurality of fins on a substrate including an NMOS region and a PMOS region adjacent to the NMOS region; forming an N-type well in the PMOS region and a P-type well in the NMOS region of the substrate; forming a protective sidewall to cover an upper portion of a sidewall surface of each fin in each of the NMOS region and PMOS region and to expose a lower portion of the sidewall surface of each fin; removing a partial width of the lower portion of the fin using the protective sidewall as an etch mask; removing the protective sidewall; and forming an isolation structure at least by oxidizing the remaining lower portion of the fin and having a top surface lower than the neighboring upper portions of the fins.