The invention relates to a thermomechanical treatment method for obtaining a high-strength and high-toughness magnesium alloy. The method comprises the following steps of: cutting a magnesium alloy ingot or a thermal deformation material into rectangular blocks; putting in a furnace, heating to 400-460 DEG C, keeping the temperature for 1-15 h, quenching in water and cooling; then, sequentially cooling by passes and multiaxially compressing and deforming along the three directions, i.e. the X-axis, the Y-axis and the Z-axis of the rectangular blocks from 300-460 DEG C; deforming and cooling to 10-120 DEG C for each pass and controlling the pass to have the true strain of 0.4-1 and the stress rate of 10<-4>-10<-1>s<-1>; aging for 0.5-10 h at 100-250 DEG C when the accumulated true strain is larger than or equal to 3 or carrying out cold deformation with a certain strain before aging to obtian the high-strength and high-toughness magnesium alloy with the tensile strength greater than 450 MPa and the elongation greater than 25 percent. The invention has reasonable process design, simple equipment requirement and convenience of operation, overcomes the problem that the delay is greatly reduced when the strength of the magnesium alloy is improved through grain refinement in the prior art and has favorable industrial application prospects.