HfTiZrTa0.5 high entropy alloy, as a refractory body-centered cubic (BCC) high entropy alloy based on elements with high combustion enthalpy, exhibits significant application potential in the field of energetic structural materials. The existing refractory BCC high entropy alloys are prone to brittle fracture under tensile loading, and it is challenging to achieve a balance between strength and toughness. In this study, the effects of heat treatment processes on the microstructure and mechanical properties of the HfTiZrTa0.5 high entropy alloy were investigated. The results indicated that a two-phase aged alloy with a BCC matrix and hexagonal close-packed (HCP) transformation-induced martensite structure was obtained after aging at 850 ℃ for 1 h followed by water quenching. Under the synergistic effect of multiple strengthening mechanisms, including the stress-induced transformation (TRIP) effect and twinning-induced strengthening, the strength and plasticity of the aged alloy were enhanced compared with the as-cast alloy. The tensile strength of the aged alloy is 864 MPa, and the elongation after break is 29.2%, which is 57 MPa higher than that of the as-cast alloy, and the elongation after break is 3.8% higher than that of the as-cast alloy.