A method of manufacturing light bulbs for us in decorative string light is provided. The manufactured glass light bulbs have a shielding layer at their bottom. The two metal conductive pins of the light bulb extend beyond the shielding layer. The manufacturing method includes using an inject molding method to form a monolithic shield layer at the bottom of the light bulb. The shape of the shield layer is manufactured to match the shape of the bulb cap used in the string light. The method provides a light bulb whose structure is easy for assembly, inexpensive and easy to manufacture, and safe in use.