The invention relates to a local gravity rigidity and assisting stay cable structural system of a long-span cable-stayed bridge, and aims to notably improve integral vertical rigidity, integral lateral rigidity and torsional rigidity of the long-span cable-stayed bridge, notably reduce the longitudinal bending moment of a bridge tower under the effect of live load, and greatly strengthen the wide applicability of the structural system in the field of long-span cable-stayed bridges for high-speed railway. The local gravity rigidity and assisting stay cable structural system comprises a bridge tower, a midspan girder and normal stay cables, wherein the upper ends of the normal stay cables are in anchoring connection with the bridge tower, and the lower ends of the normal stay cables are in anchoring connection with the midspan girder. The local gravity rigidity and assisting stay cable structural system is characterized in that the midspan girder consists of a midspan central region spanning girder and ordinary standard section girders on two sides; a local ballast weight is arranged on the midspan central region spanning girder; and a plurality groups of assisting midspan stay cables and assisting sidespan stay cables, of which the upper ends are in anchoring connection with the bridge tower, are arranged on the midspan central region spanning girder; the lower ends of the assisting midspan stay cables are in anchoring connection with the midspan central region spanning girder; and the assisting sidespan stay cables are opposite to the assisting midspan stay cables, and the lower ends of the assisting sidespan stay cables are in anchoring connection with the ordinary standard section girders on the side of a sidespan.