A flash preventing substrate and a method for fabricating the same are proposed. A core defined with a plurality of substrate units is prepared. A circuit patterning process is performed to form circuit structures on the core corresponding to the substrate units, plating buses between the adjacent substrate units and electrically connected to the circuit structures, and a molding ring surrounding all the substrate units. The molding ring is located at a position predetermined for contacting the substrate with a mold. A solder mask layer covers the circuit structures, the plating buses and the molding ring, and is formed with a plurality of openings therein, such that predetermined portions of the circuit structures are exposed via the openings and serve as electrical input/output connections. During a molding process, the mold can tightly abut against the solder mask layer covering the molding ring to prevent outward flashes of an encapsulating material.