The invention relates to a bridge deck slab transverse unequal-strength steel-concrete combined bridge deck system and a construction method thereof and belongs to a novel bridge deck structure system applied to a large-span suspension bridge and a cable-stayed bridge. The combined bridge deck system comprises ultrahigh-performance concrete on the outer side of the bridge deck, common concrete on the inner side of the bridge deck and bridge deck steelwork, wherein the ultrahigh-performance concrete is paved within 1/8-1/4 of bridge width of the outer sides along the width direction of the bridge deck, and the common concrete is paved within the rest width range. The bridge deck steel structure adopts the bridge deck system commonly used in a flexible large-span bridge, such as a common flat steel box girder and a steel truss, and is connected to integrally and jointly bear force by antishear connectors or antishear connecting measures, therefore, stress level of the steelwork is lowered, integral rigidity of the bridge deck is improved and steel usage of the bridge deck is reduced. The advantages of a common combined bridge deck system and the high-performance concrete are integrated, and the bridge deck slab transverse unequal-strength steel-concrete combined bridge deck system has the advantages of high strength and rigidity, light dead weight, good anti-cracking performance and easy construction and the like and has wide application prospect in the field of the large-span bridge structure.