Abstract: The high-temperature thermal exposure experiment of aluminum (Al) alloy is a method used to characterize the real-world performance of components in high-temperature environments. In this study, thin-walled Al–Cu heat-resistant aluminum alloy structures were additively manufactured through the wire-arc directed energy deposition (WA-DED) process. A systematic analysis was conducted to investigate the microstructural changes and uncover the origins of mechanical degradation under 300°C thermal exposure. Following T6 heat treatment, the WA-DED 205C alloy exhibited a dense distribution of nano-sized θ^'', θ^', Al3Zr and T (Al20Cu2Mn3) phases within the grains, whereas the γ (Al7Cu4Ni) phase formed during solidification remained at the grain boundaries. Following thermal exposure at 300°C for 24 h, the majority of θ^'' within the grains were supplanted by θ^', a subset of these θ^' subsequently transitioned to θ, while σ (Al5Cu6Mg2) phases concurrently precipitated. As thermal exposure reached 100 h, the θ^' phase exhibited increased particle size alongside reduced number density, whereas the θ phase showed enhanced density and the σ phase was no longer present. The Al3Zr and T phases within the grains, and the γ phase located at the grain boundaries, showed no significant changes in morphology or size during thermal exposure process. Furthermore, after thermal exposure at 300°C for 24 h, the microhardness of the T6 condition specimen experienced a sharp decline from 149.6 HV to 98.2 HV and the yield strength (YS) dropped from 353 MPa to 213 MPa. Following 100 h of thermal exposure, the microhardness decreased to 91.8 HV and the YS dropped to 196 MPa. The alloy’s tensile properties appeared to stabilize, with a YS retention rate of 56%.