The invention relates to a blast furnace shell welding line destressing device and a method. A heating device is arranged on the outer wall of a shell for covering a welding line and a heat affected area thereof; an insulation layer covers the heating device; a temperature measuring device is arranged on the inner wall of the shell in the welding line area; and the temperature measuring device and the heating device are connected with a temperature control system. The destressing method comprises the following steps: firstly, the heating device, the insulation layer and the temperature measuring device are arranged on the shell of a blast furnace; then, the heating is performed according to the following temperature curve; the temperature is increased to 300 DEG C; then, the temperature is increased to 620+-20 DEG C at a speed of not more than 80 DEG C; the temperature is kept by 1.5 h; then, the temperature is cooled below 300 DEG C at a speed of not more than 60 DEG C; and finally, the temperature is cooled to the normal temperature; and the temperature measuring device, the heating device and the insulation layer are removed. The heating device can cover the whole welding line of the shell and the heat affected area; the temperature measuring device monitors the temperature of the welding line; the temperature control system controls the temperature; and the temperature is increased or decreased according to specific temperature curves, so that the whole welding line is synchronously heated and destressed, and the treatment effect is better.