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High-entropy alloys (HEAs), recently emerging alloys with numerous excellent mechanical performances, may have a wide application prospect in impact engineering. The ballistic impact response of Fe40Mn20Cr20Ni20 HEA was investigated under various loading conditions. Ballistic impact tests with spherical projectiles and 87 type 5.8 mm small caliber bullets (DBP87 bullets) were conducted on 10 mm thick Fe40Mn20Cr20Ni20 HEA plates with varying impact velocities, compared with 20Mn23AlV steel (high manganese low magnetic steel). The relationship between microstructural details and aspects of ballistic behavior governing performance was established through experimental explorations and theoretical models. According to the findings, dense dislocation structures led to distinguishing work hardening in the HEA, and the strain-hardening capacity of the HEA enhanced dramatically with increasing strain rate under dynamic tension. Meanwhile, under 500 m/s impact velocity, twinning and microbanding had outstanding strain-hardening capabilities for the current HEA, and the cooperation of the dislocation slip and stacking faults was critical for strain hardening in the HEA when the impact velocity was increased to [Formula: see text], whereas only a small amount of dislocation sliding and twinning occurred during the dynamic deformation process of 20Mn23AlV steel at different impact velocities. These findings demonstrated that the outstanding strain-hardening capabilities of Fe40Mn20Cr20Ni20 HEA made it a promising candidate for ballistic impact engineering compared with 20Mn23AlV steel.