Abstract: To identify the compressive stability of the steel–ultra-high performance concrete (UHPC) composite deck structures in the light-weight composite girder systems of cable-stayed bridges, based on the test parameters including steel flange thickness, UHPC layer thickness, and stiffener shapes, five steel and steel–UHPC composite deck specimens with a geometric scale ratio of 1∶2 were designed and fabricated, and their compressive stability was investigated experimentally. The results show that: Steel specimens exhibit branch point instability mode, and the composite specimens possess high post-buckling bearing capacity with extreme point instability mode due to the strengthening effect of UHPC layer and the internal stress/force redistribution characteristics between steel and UHPC layer on the section where the steel is subjected to local buckling. Setting a 30 mm UHPC layer on the steel specimens can increase their elastic axial stiffness and compressive stability bearing capacity by 102.1% and 119.7%, respectively. Compared with the steel specimens with the same transformed cross-sectional area, the compressive stability bearing capacity of the steel-UHPC composite specimens can still be increased by 22.2%. The T-rib, which has the same cross-sectional area and moment of inertia around the strong axis as the U-rib, has better local stability; however, due to its lower torsional performance, the steel-UHPC composite specimens reinforced with T-ribs suffer overall flexural-torsional buckling failure, but the bearing capacity is similar to those of the specimens reinforced with U-ribs. A calculation formula for the bearing capacity of steel–UHPC composite decks under compressive stability was proposed based the different mechanical behaviors of steel and steel–concrete composite compressive members, and the applicability of formula was verified by the experimental results.