A semiconductor device and a method for forming the semiconductor device are provided. The method includes providing a layer to-be-etched including a first sub-trench region and a second sub-trench region. The method also includes forming a first mask layer over the layer to-be-etched and a second mask layer over the first mask layer, and forming a first sub-trench disposed over the first sub-trench region in the second mask layer. In addition, the method includes forming a first divided trench in the first mask layer and forming a second sub-trench disposed over the second sub-trench region in the second mask layer. Further, the method includes forming a first divided filling layer in the first divided trench, and forming a first middle trench in the first mask layer. The first divided filling layer divides the first middle trench in a second direction.