The invention discloses an electric arc automatic-welding welding fume collecting device and a method for testing welding fume amount. The collecting device comprises a fume collecting box, wherein the fume collecting box is provided with a box door, the top of the fume collecting box is provided with a filtering film, and a left shaft and a right shaft the inner ends of which are provided with top cones symmetrically penetrate through the wall of the box; the outer end of the left shaft is connected with a motor, and a device which is connected with and used for clamping a welding gun through an adjustable transverse guide column and a vertical guide column is arranged in the fume collecting box. The method for testing welding fume amount comprises the following steps: generating and precipitating welding fume by the device; then collecting precipitated fume, microsplash, welding slag, and the like with velvet paper and absorbent cotton; further separating the fume, the microsplash and the slag with microsize; and obtaining the fume amount in unit time by calculation. The invention improves the stability in the process of electric arc welding by automatic welding, reduces the influence of human factors, collects fume in the collecting box after natural precipitation and can realize more complete separation of the welding fume, the microsplash and the smelting slag.