The invention relates to an auxiliary desulphurizing device implemented by powder injection in torpedo and a process thereof. On the basis of top powder injection equipment, two air bricks (4) are symmetrically arranged at the bottom part or the bottom lateral part of a torpedo (8), each of which is communicated with a relief air tank (3) via a pipeline, and the head part of the bottom part of the relief gas tank (3) is communicated with a nitrogen gas or argon gas source (1) via a valve (2). The air brick (4) is laid by use of slit type air bricks, and has an outlet diameter of 40 to 100 mm, a slit width of 0.1 to 0.3 mm and the number of slit of 10 to 40; and the ventilation surface thereof is parallel to the bottom surface of the torpedo (8). The process comprises the following steps: carrying out top powder injection, while injecting powder by use of nitrogen gas or argon gas via the two air bricks (4) arranged on the bottom part or the bottom lateral part of the torpedo (8), wherein, the powder injection pressure and the air permeability are controlled to be 0.1 to 1.0 Mpa and 15 to 50 m<3>/h, respectively. The process can reduce the area of dead zone, shorten mixing time, and improve dynamic conditions of metallurgical reactions; and has high desulphurizing efficiency.