A low-back-pressure penetrating arc welding apparatus and a welding method using the same are disclosed. Through vacuuming a central vacuum chamber of a low-back-pressure protection device by a suction device, a plume of a penetrating arc located in the central vacuum chamber, as well as a back side of a keyhole molten pool, are maintained in a negative-pressure vacuum state relative to an argon gas ambience. A pressure gradient in the keyhole molten pool is thus generated that points from a front side of the keyhole molten pool to the back side of the keyhole molten pool. The pressure gradient thus further enhances a stiffness of the front side of the keyhole molten pool as well as a stability of the penetrating arc, resulting in an enhancement in the piercing capability of the penetrating arc, without changing a force distribution and a temperature gradient of the keyhole molten pool.