The AlCrCoFeNi_2.1 high-entropy alloy (HEA) was prepared by the spark plasma sintering method at different temperatures. The microstructure, corrosion resistance, and mechanical properties of this HEA were investigated. Results show that the maximum relative density of the AlCrCoFeNi_2.1 HEA can reach 99.18% after sintering, and the HEA is mainly composed of body-centered cubic (bcc) and face-centered cubic (fcc) phases with the phase fraction of 20.6% and 79.4%, respectively. Compared with that in fcc phase, the fraction of recrystallized and deformed microstructures in bcc phase of AlCrCoFeNi_2.1 HEA is higher. In addition, the bcc phase can be easily corroded in 3.5wt% NaCl solution. The pressure recovery rate of bcc and fcc phases is decreased and the hardening effect is enhanced with increasing the strain rate. The higher ultimate tensile strength of AlCrCoFeNi_2.1 HEA after sintering at 1050 °C can be achieved due to the grain size strengthening, solid solution strengthening, and good interface bonding between HEA particles. The failure mode of AlCrCoFeNi_2.1 HEA includes the brittle fracture of bcc phase and the ductile fracture of fcc phase.