It is of substantial scientific significance and practical value to reveal and understand the multiscale mechanical properties and intrinsic mechanisms of medium-entropy alloys (MEAs) under high strain rates and pressures. In this study, the mechanical responses and deformation mechanisms of an equiatomic CoCrNi MEA are investigated utilizing magnetically driven ramp wave compression (RWC) with a strain rate of 10⁵ s^-1. The CoCrNi MEA demonstrates excellent dynamic mechanical responses and yield strength under RWC compared with other advanced materials. Multiscale characterizations reveal that grain refinement and abundant micromechanisms, including dislocation slip, stacking faults, nanotwin network, and Lomer–Cottrell locks, collectively contribute to its excellent performance during RWC. Furthermore, dense deformation twins and shear bands intersect, forming a weave-like microstructure that can disperse deformation and enhance plasticity. On the basis of these observations, we develop a modified crystal plasticity model with coupled dislocation and twinning mechanisms, providing a relatively accurate quantitative description of the multiscale behavior under RWC. The results of simulations indicate that the activation of multilevel microstructures in CoCrNi MEA is primarily attributable to stress inhomogeneities and localized strain during RWC. Our research offers valuable insights into the dynamic mechanical responses of CoCrNi MEA, positioning it as a promising material for use under extreme dynamic conditions.