The invention discloses a welding tool of a sheet electrode of a capacitor. The welding tool comprises a base, a positioning block, an air cylinder mounting rack, an air cylinder and a compressing piece, wherein a copper plate fixing bar is arranged above a relatively low surface of the front part of the base, and a copper plate mounting groove is formed on a relatively high surface of the rear part; the positioning block is an integrated part and is composed of a copper plate positioning part below and an electrode positioning part above; push rod mounting holes are formed on the copper plate positioning part, and push rods can move front and back in the push rod mounting holes to adjust the position; two sheet grooves are formed at the front end of the electrode positioning part, and the sheet grooves are matched with an sheet electrode in size; and each of the sheet grooves is provided with a cylindrical positioning bump. When the welding tool of the sheet electrode disclosed by the invention is used for mounting and positioning the sheet electrode, a through hole on the sheet electrode sleeves the bump, and a compressing sheet compresses the surface of the sheet electrode to fix and position the electrode, and the compressing sheet and nuts are not needed to fix the sheet electrode, so that the welding efficiency is greatly improved.