A method for fabricating semiconductor device includes the steps of: providing a substrate, wherein the substrate comprises a first semiconductor layer, an insulating layer, and a second semiconductor layer; forming an active device on the substrate; forming an interlayer dielectric (ILD) layer on the substrate and the active device; forming a first contact plug in the ILD layer to electrically connect the active device; and forming a second contact plug in the ILD layer and the insulating layer after forming the first contact plug.