Abstract: Ultra-high performance concrete (UHPC) and recycled aggregate concrete (RAC), with a low carbon footprint, belong to the "Low Carbon Concrete". A portion of RAC on the tensile side or side wall of the RAC beam was replaced with UHPC to form a "green and low carbon" UHPC-RAC composite section to improve mechanical pro-perties. The precast UHPC-RAC composite beam was fabricated by precast technology. The influences of tensile UHPC thickness, roughness of UHPC-RAC joint surface and UHPC height of side wall on failure mechanism, bearing capacity, deformation and initial stiffness of precast UHPC-RAC composite beams were analyzed by four-point flexural tests, then the calculation formula of bearing capacity were proposed. The results show that, comparing precast UHPC-RAC composite beams with RAC beams, the UHPC-RAC bonding surface traversed by the stirrups with the increase of UHPC thickness on the tensile side limits the peeling off of UHPC after cracking. The increasing roughness of the interface further retards the extension of horizontal cracks and improves the initial stiffness about 16.6%. With the failure of steel fiber pulling out in the UHPC on the tensile side, the load-displacement curves decrease obviously. The precast composite beams still have a high residual strength when the compression concrete is crushed. Compared with RAC beams, the cracking load and ultimate load of precast UHPC-RAC composite beams are increased by 63.1% and 22.9%, respectively, and the section flexural stiffness and initial stiffness are signifi-cantly improved. In the composite section, reinforcement, UHPC and RAC work collaboratively, and the strain changes linearly along the section height, conforming to the assumption of plain section. After equivalent section stress distribution, the calculation formula of the bending capacity of the precast UHPC-RAC composite beams is deduced, and the calculated results are in good agreement with the experimental values.