Lithium aluminum (Al–Li) alloy is a new-generation lightweight alloy material used in aerospace applications. In this study, arc additive manufacturing was used to fabricate aluminum–lithium–silicon alloy. A comparison was made between the microstructure and mechanical properties of the alloy in the as-deposited sample and the T6 heattreated sample. The experimental results show that the average size of the α–Al phase in the single-pass thin-walled deposited aluminum-lithium-silicon alloy was approximately 21.2 μm. SEM and XRD results revealed the presence of a large amount of micrometer-scale eutectic Si phase and eutectic phases containing Cu and Li in the microstructure. After T6 heat treatment, the coarse Al₂Cu phase and some eutectic silicon structures gradually dissolved into the matrix. The hardness of the additive sample increased from 96HV to 138HV, representing a relative increase of 43.8%. The tensile strength in the 0°, 45°, and 90° directions was 402 MPa, 350 MPa, and 330 MPa, respectively. The as-deposited samples exhibited tensile strengths of 160 MPa, 134 MPa, and 142 MPa in three directions, whereas the T6 heat-treated samples showed significantly improved mechanical properties.