Modification of 6061 aluminum alloy was conducted through composite addition of cerium-rich rare earths and Al-Ti-B. Results show that the composite addition of Al-Ti-B and Ce/La element at a specific ratio notably promotes the refinement of the alloy's grains. Ce and La elements are combined with Si and other elements to form rare earth phases, improving the morphology and distribution of precipitates and mitigating the adverse effects of β-Fe phases on the microstructure and mechanical properties of alloy. However, excessive rare earth content poses challenges; it not only leads to a decrease in Mg-Si strengthening phase by binding with Si but also promotes the formation of larger or numerous rare earth phases that may act as initiation points for cracks, thereby impeding the improvement of the structure and performance of alloy. The composite addition of cerium-rich rare earths and Al-Ti-B not only preserves the strength of the alloy but also significantly enhances the plasticity of the 6061 as-cast alloy. At a composite addition ratio of Al-Ti-B:RE=2:1, the newly developed 6061-RE aluminum alloy exhibits increased average elongation by 50% and 45% in its as-cast and homogenized states, respectively, compared to the baseline 6061 alloy, facilitating subsequent deformation processing. After solution treatment at 540 °C for 1 h and aging at 180 °C for 5 h, the average ultimate tensile strength and yield strength of 6061-RE alloys reach 313.2 and 283.1 MPa, increased by 12.3% and 14.5% compared with those of the original alloy, respectively, and the average elongation is improved by 41%.