•The high-Li-content in-situ TiB2/Al-Li-Cu composite was successfully synthesized.•The precipitation behavior of the novel cast composite was studied.•The hardness change was dominated by the evolution of T1 (Al2CuLi) and δ′ (Al3Li).The microstructural evolution and mechanical properties of the cast in-situ TiB2/Al-3.3Li-1.2Cu composite during heat treatment were systematically investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and Vickers hardness (HV). The nano TiB2 particles mainly agglomerated along the grain boundaries, showed a clean and clear interface between matrix. Much fine δ′ (Al3Li) particles were distributed homogeneously after quenching, and its size increased with the increase of aging time. Primary θ′ and T1 precipitates were visible in the under-aged (UA) state, which contributes to a rapid increase in hardness. And the plate-like T1 precipitates of very high aspect ratio were precipitated with prolonging aging time. It was worth mentioning that the thickness of this plate-like T1 precipitates was stable as the aging proceeded. The studied composite, which was largely strengthened by the δ′ and T1 phases in the peak-aged condition, had a strong age-hardening response. After aging at 175 °C for 214 h, a peak-hardness, which was nearly double than that of the as-quenched, was obtained.