A manufacturing method of a molded image sensor packaging structure with a predetermined focal length and the structure using the same are disclosed. The manufacturing method includes: providing a substrate; providing a sensor chip disposed on the substrate; providing a lens module set over the sensing area of the chip to form a semi-finished component; providing a mold that has an upper mold member with a buffer layer; disposing the semi-finished component into the mold to form a mold cavity therebetween; injecting a molding compound into the mold cavity; and after transfer molding the molding compound, opening the mold and performing a post mold cure process to cure the molding compound. The buffer layer can fill the air gap between the upper surface of the lens module and the upper mold member, thereby preventing the upper surface of the lens module from being polluted by the molding compound.