Abstract: Aiming at the problems of poor plasticity and toughness of as-cast Mg-Y-Zn alloys containing 18R-LPSO phase and limited improvement of mechanical properties after deformation, the effects of solid solution, as-cast +ECAP extrusion and solid solution+ECAP on mechanical properties of Mg-3.74Y-1Zn-0.5Al (at%) alloy were studied. The results show that the solution treatment induces 18R→14H-LPSO phase transformation and the Al (Y, Zn) phase compound particles are broken and refined, which improves the plasticity of the alloy, but the strength is almost unchanged.Cast+ECAP extrusion leads to the fragmentation of 18R-LPSO and Al(Y, Zn) phases, and induces recrystallization of a large number of α-Mg grains.Solid solution+ECAP extrusion makes 14H-LPSO bend or kink, fine Al (Y, Zn) particles are uniformly dispersed, and only a few of α-Mg grains recrystallize, and its strength, ductility and toughness are much higher than those of cast+ECAP alloy.The main reasons for the improved performance are the large formation of 14H-LPSO phase with good strength-plasticity-toughness matching, the suppressed recrystallization of α-Mg matrix and the uniform dispersion of fine Al(Y, Zn) particles.