Microalloying is an effective and feasible strengthening strategy for Al alloys. However, the influence of Cd, which is the most important trace element of ZL205A, on precipitation hardening is not fully understood. On this basis, the effects of Cd on the microstructure and tensile properties of the ZL205A alloy were systematically investigated. The results show that the microalloying of Cd significantly enhances the strength by ~130 MPa. Microstructure analysis indicates that the main precipitate of the T6 ZL205A alloy is the θ″ phase, and the addition of Cd element promotes the θ″→ θ′ phase transformation, resulting in θ′ becoming the dominant precipitate. This makes the alloy precipitation strengthening mechanism change from dislocation shearing through the θ″ phase to dislocation bypassing the θ′ phase, thereby effectively enhancing the strengthening effect. On the basis of the STEM-EDS characterization, it is suggested that Cd promotes the θ″ → θ′ phase transformation through two synergistic mechanisms: ① Cd has high vacancy binding energies, leading to the formation of Cd-Cu-vacancy clusters, providing Cu atoms for transformation; ② the Cd-vacancy pairs dissociate at the temperature of the θ′ phase formation and release vacancies, which increases the diffusion rate of the Cu atoms in the Al matrix, thus promoting the transformation of θ″ to θ′.