The invention relates to a welding device, in particular to a mechanical tracking device of a tank-type double-gun horizontal circular/longitudinal seam welder. The device mainly comprises a tracking roller (1), a roller shaft (2), a roller bracket (3), a left-handed and right-handed screw rod (4), a first thin lock nut (5), a coupling screw (6), a downward spring adjusting screw rod (7), a support sleeve lock screw (8), a tracking extension spring (9), a spring bushing (10), a second thin lock nut (5), an upward spring adjusting screw rod (12), an elevating cylinder (13), a linear slide rail and slide block (14), a welding gun (15), a three-dimensional crossed fine tuning mechanism (16) and the like. The tracking of circular/longitudinal seam welding can be realized. Simultaneously, seams are detected by a mechanical method without disturbing normal work of a welder or causing adverse effect on an electric arc of the welder, so that operation is more convenient and cost is low.