The forward extrusion experiments of homogenized AZ80 magnesium alloy were conducted in the extrusion temperature range of 300°C to 420°C and extrusion ratios between 15 and 75 to study the effect of plastic deformation on the mechanical properties and microstructure. The microstructure and mechanical properties of extrudate were measured by tensile tests and optical microscopy. The results demonstrated that the alloy grains were small, and small amounts of black hard and brittle second-phase precipitated at 330°C. When the extrusion temperature was up to 390°C, the grain size increased significantly, but the second phase precipitation became continuous. And then, in the case of the extrusion ratio of 60, the tensile strength of the alloy reached the peak value of 390 MPa. Inhomogeneous precipitation of the second-phase increased with the increasing of extrusion temperature. At the same temperature, the tensile strength increased firstly and then decreased with increasing extrusion ratio. With the gradual increase of the grain refinement, the dispersed precipitates increased and the tensile strength and plasticity reached their peaks when the extrusion temperature was 390°C. As the grain grew, the second phase distribution became inhomogeneous, and the strength and plasticity gradually decreased.