A semiconductor device and its fabrication method are provided. The method includes providing a layer to be etched; forming a first mask layer on the layer to be etched; forming a first trench and a second trench in the first mask layer; forming a blocking layer over the first mask layer, where a portion of the blocking layer is formed in a first portion of the first trench and a first portion of the second trench; forming a first dividing layer in a first blocking opening to divide the first trench along a first direction; when forming the first dividing layer, forming second dividing layers on two sidewalls of a second blocking opening and arranged along the first direction, where the second dividing layers divide the second trench along the first direction; and after forming the first dividing layer and the second dividing layers, removing the blocking layer.