Eutectic high-entropy alloys have excellent strength and plasticity, and also have the good casting performance of traditional eutectic alloys, which is of great significance for the practical application of high-entropy alloys. However, how to further improve the strong plasticity of eutectic high-entropy alloys has become a research hotspot in the field of high-entropy alloys. In this paper, an Al21.5Co19.5Fe9.5Ni50 eutectic high-entropy alloy was used as the research object, and the influence of thermomechanical treatment on the microstructure and tensile mechanical properties of the alloy was investigated. The deformation mechanism of the alloy under thermomechanical treatment conditions and its influence on the mechanical properties of the alloy were also elucidated in relation to the influence of the microstructure and phase structure of the alloy on its strain hardening ability. The results show that after thermomechanical treatment, the alloy transforms from a eutectic lamellar structure to a nearly complete equiaxial crystalline structure, and the L12 phase precipitates out of the FCC phase. During the tensile deformation of the alloy after thermomechanical treatment, with increasing strain, the dislocation density in the FCC phase increases, and a stress-induced martensitic phase transformation occurs in B2 phase to form the L10 phase with an interlocking twin structure. Finally, under the dual strengthening mechanism of the FCC phase and the B2 phase, the alloy exhibits a higher yield strength (551 MPa) and elongation at break (10.2% ), and the work hardening rate curve shows significant changes.