AlCoCrFeNi2.1 eutectic high-entropy alloys effectively overcome the inherent strength-ductility trade-off bottleneck of single-phase high-entropy alloys with FCC/BCC structures via the coordinated deformation of soft and hard phases. To address the key scientific issues of the microstructure evolution and performance regulation of the alloy under nonequilibrium solidification conditions, this study adopted 35 m drop tube containerless solidification technology to realize deep undercooling and rapid solidification of the alloy. Spherical samples with particle sizes ranging from 100 to 2 800 μm were prepared, and the coupled regulatory mechanism of particle size on the microstructure evolution and mechanical properties of the alloy was systematically clarified. The results show that all the drop-tube samples maintain a dual-phase structure of FCC +B2. With decreasing particle size, the evolution path of the alloy microstructure is as follows: radial gradient structure (2 800~1 500 μm, from primary dendrites+anomalous eutectic on the surface to regular lamellar eutectic in the center)→dendrite refinement and fragmentation (1 200~520 μm)→complete anomalous eutectic (<520 μm). The evolution of the microstructure directly leads to the differential response of the mechanical properties: large samples exhibit ahardness gradient of low center and high periphery, whereas the hardness of small samples (1200~100 μm) increases linearly with decreasing particle size (307~324 HV).