To reduce the heavy self-weight and simplify the construction process of concrete arch bridges, a new type of structure of concrete-steel truss web arch was proposed, which are composed of concrete upper and lower flanges and steel truss web members. The Wumengshan Bridge is the first such composite arch bridge, which is under construction. UHPC has good durability and corrosion resistance and higher strength than conventional concrete, the self-weight of such composite arch can be furtherly reduced If UHPC is used to replace conventional concrete, and the service life od arch bridge can be extend, which has significant economic benefits. To investigate the mechanical behavior of UHPC-steel truss web composite Arch Bridge, a trial-design on such composite arch bridge is carried out based on the prototype of the Wumengshan Bridge. The trial design results show that the sectional dimensions of arch rib can be greatly reduced using UHPC. Compared with concrete-steel truss web composite arch rib, the dead load of UHPC-steel truss web arch rib can be reduced by 20.9%. The bending moment and axial force at the arch seat were reduced by 22.1% and 35.9%, respectively, and those at the vault were reduced by 21.2% and 61.3%, respectively. The horizontal thrust at the arch seat was reduced by 21.2%.