Semiconductor devices and fabrication methods thereof are provided. An exemplary fabrication method includes providing a semiconductor substrate; forming a plurality of fins on a surface of the semiconductor substrate; forming an isolation flowable layer covering the plurality of fins over the semiconductor substrate; performing a first annealing process to turn the isolation flowable layer into an isolation film; and forming first well regions and second well regions in the fins and the semiconductor substrate. The second well regions are at two sides of the first well regions and contact with the first well regions; the first well regions have a first type of well ions; the second well regions have a second type of well ions; and the first type is opposite to the second type in the conductivities.