A method to form a fin structure on deep trenches (DTs) for a semiconductor device includes the following steps: A buried oxide layer (BOX) having the DTs, and silicon polies in the DTs is provided. A fin on the BOX and the silicon polies having poly fences is provided. A first mask is disposed on the fin. A liner is disposed on the BOX and the first mask, wherein the liner has a first part above the fin, a second part at lateral sides of the fin and a third part on the DTs and the BOX. A second mask is disposed on the first and the second parts of the liner. The second mask and the third parts of the liner are removed to reveal the first and the second parts of the liner. The poly fences are removed and spacers at the lateral sides are formed.