The invention discloses a forming method of a packaging structure. The method comprises the steps of: forming a first shielding layer which wraps the non-functional surfaces and side wall surfaces ofa plurality of semiconductor chips after the functional surfaces of the semiconductor chips are bonded on a carrier plate; forming a second shielding layer on the first shielding layer; forming a plastic package layer on the second shielding layer and the carrier plate between the semiconductor chips; stripping the carrier plate to form a pre-sealing panel, wherein the back surface of the pre-sealing panel exposes the functional surfaces of the semiconductor chips; and forming an external contact structure connected with bonding pads on the back surface of the pre-sealing panel. According to the invention, the second shielding layer is formed on the first shielding layer, and the second shielding layer can cover places with non-uniform thickness and poor edge coverage in the first shielding layer, so that the whole shielding layer formed by the first shielding layer and the second shielding layer is complete, and the shielding effect is improved.