The invention discloses an automatic forming apparatus of large-sized thin injection molding parts.Specifically, a plurality of injection molding ports, a small-diameter screw and a high-tonnage bench are arranged all in a forming cavity, they are combined so that hot melt adhesive can be completely applied to fill the narrow forming cavity, the poor formation problems such as formation of weld lines are avoided, a large-size thin injection molding part that is made has flat, smooth and burr-free surface, and the precision is high.The invention also discloses a forming method of large-sized thin injection molding parts; a multi-stage heating mode is used specifically to multiply heat a fiberglass and polycarbonate system, and it is possible to avoid fiber floating in the injection molding process, by changing many times the temperature of a mixture system and using a thermal expansion coefficient difference between two components.It is possible to effectively improve forming effect of thin injection molding parts so that the thickness of the injection molding parts is reduced and quality indexes such as mechanical properties and appearance can be maintained, thereby enabling a reduction in the size of an injection molding machine, a reduction in energy consumption, an improvement in the thinness of the injection molding parts and the like.