Abstract: This paper proposes a novel ultra-high performance concrete （UHPC） panel-steel keel composite wallboard. Designed with steel keels as the primary load-bearing components， the wallboard utilizes UHPC for its exterior panels to enhance impermeability and durability. Glass fiber-reinforced polymer （GFRP） connectors are employed to reduce heat conduction and improve structural integrity between the UHPC panels and the steel keel framework. This design meets the requirements for external wallboards in frame structures of ultra-low energy consumption buildings. A full-scale composite wallboard was fabricated and subjected to an out-of-plane bending test under a uniformly distributed load with four-point supports， complemented by finite element simulation.The results showed that the GFRP connectors had a significant impact on the integrity of the composite wallboard， resulting in a 30% increase in bending stiffness and a 30% reduction in both the mid-span deflection and the steel keel’s mid-span bending moment. When the load reached 14 kN/m²， the steel keel of the test wallboard reached the yield stress， and fine cracks with widths not exceeding 0.1 mm appeared on the UHPC panels. At the load of 25 kN/m²， the supports failed； the slope of the load-deflection curve decreased to 29% of its initial value， but it had not yet entered the descending section. A limited number of through-thickness cracks were observed in the UHPC panels near the four corner supports and at the mid-span of each edge. Except for some irreversible lateral deformation in the corner GFRP connectors， all other components remained intact. These findings indicate that the proposed UHPC panel-steel keel composite wallboard can meet the mechanical requirements for prefabricated exterior wallboards in frame structures.