A fabrication method of a layer structure for mounting a semiconductor device is provided, which includes the steps of: providing a base material, wherein the base material has a conductive layer having a first surface having a plurality of first conductive elements and an opposite second surface having a plurality of second conductive elements, and a first encapsulant formed on the first surface of the conductive layer for encapsulating the first conductive elements; partially removing the conductive layer to form a circuit layer that electrically connects the first conductive elements and the second conductive elements; and forming a second encapsulant on a bottom surface of the first encapsulant for encapsulating the circuit layer and the second conductive elements, thus reducing the fabrication difficulty and increasing the product yield.