The invention discloses a deepening and shaping method for a Mg-alloy tailor-welded blank which includes the following technique flows: 1) preparing a blank material: a lubricant is coated on the blank material of the Mg-alloy tailor-welded blank; 2) preheating a die: the die is heated to a preset temperature and the blank material is put in the die after the temperature detected by a thermoelectric couple reaches a preset value; the blank material is heated together with the die; 3) heating the blank material: after the blank material and the die are heated together for 5 to 10 minutes, deepening and shaping is prepared; 4) deepening and shaping: the deepening and shaping speed is controlled in a preset range and deepening and shaping is carried out; 5) finishing the deepening and shaping, after the deepening and shaping is finished, a workpiece is taken out. The method realizes the uniform deformation during the deepening and shaping process of the Mg-alloy tailor-welded blank, avoids the defects of cracking or serious deviation of a welding line and improves the product quality; a deepening and shaping device for the Mg-alloy tailor-welded blank can lead the blank material to realize grads temperature field distribution; besides, the temperature control method is simple which can realize different temperature grads fields only by changing the number and distribution of heating rods.