The compressive property and microstructure of Mg-3Al-6Zn-2Y alloy at the strain rates of 0.001, 900, 1300 s-1 were studied. The hardening rate, ultimate strength, and yield strength increased with strain rate increasing, while the reinforcement phases of Al2Y and Al12Mg17 and the transition phase MgZn2 decreased. The {102}<100> twinning occurred with higher density of dislocation which tangled and piled up on the interface of crystal twin. The weakening effect on intensity caused by decreasing of reinforcement phases competes with the intensifying effect caused by the higher density of dislocation piling up on the interface of crystal twin.