The reverse temperature field extrusion process as improved extrusion process for producing microcrystal magnesium alloy includes the following steps: preheating extrusion mold, setting magnesium alloy blank into the extrusion mold, controlling the mold temperature the same as or 30-50 deg.c higher than that of blank surface and the temperature difference between the core of the blank and the surface of the blank in 20-400 deg.c, and extrusion at extrusion ratio of 2-64 and extrusion ram speed of 20-30 mm/s. Extruding magnesium alloy in the said reverse temperature field and at extrusion ratio of 4-6 can fine the crystal particle to below 10 micron, or below 5 micron optimally. Meanwhile, the extruded magnesium alloy has obvious raised plasticity and unit extension over 20 %. The present invention may be used in producing magnesium alloy rod, plate, section bar and pipe.