A fundamental understanding of the mechanical behavior of materials subjected to dynamic loading is critical for developing outstanding structural materials. In this paper, we used laser-induced high-velocity (500 ∼ 800 m/s) micro-projectile impact experiments to measure the impact response of 100 nm thick CoCrNi medium entropy alloy (MEA) nanofilm. The results revealed that the CoCrNi MEA nanofilm exhibits high specific penetration energy (up to 0.882 MJ/kg) and excellent impact resistance, significantly surpassing traditional protective materials. The specific penetration energy of CoCrNi MEA nanofilm is approximately 1.8 to 2.2 times that of steel and 1.2 to 1.4 times that of Kevlar composite plates. Based on the post-impact analysis, we observed abundant energy dissipation pathways including multiple cracks, bending of cracking-induced petals, mechanical twins, and, of particular note, amorphization for the nanocrystalline CoCrNi MEA nanofilm. Such solid-state amorphization stemming from severe lattice distortion activates a new mechanism for impact energy dissipation. The versatility and synergy of these deformation mechanisms contribute to the exceptional protective performance of the nanocrystalline CoCrNi MEA nanofilms. The specific penetration energy of the nanocrystalline CoCrNi MEA nanofilm is about 21% higher compared to that of the amorphous CoCrNi MEA nanofilm due to the additional energy dissipation mechanisms arising from mechanical twins and amorphization. This study provides valuable physical insights into the impact resistance and energy dissipation mechanisms of MEA nanofilm and highlights its potential as a high-performance coating to enhance the surface integrity and reliability of equipment subjected to high-speed collisions of solid particles.