The invention discloses a rubber pad forming method of a concave surface semi-arc reinforced flange sheet metal part. A mold is included, the mold comprises a bottom plate, a mold body, a cover plateand an auxiliary cushioning block, the shape of the working face of the mold body is in accord with the concave surface shape of a to-be-molded metal sheet part, the shapes of the auxiliary cushioningblock and the working face of the mold body are matched, the height of the shape of the auxiliary cushioning block is lower than the mold body, the mold body and the auxiliary cushioning block are connected with the bottom plate, wools are fixed to the bottom plate, the cover plate is pressed downwards by using hydraulic molding equipment, suspended sheet materials are pressed by rubber pads to adhere to the working surface of the mold and the upper surface of the auxiliary cushioning block to form a transition work piece with an L-shaped inner flange, the transition work piece is placed on another flanging mold, and therefore the outer side flange of the transition work piece is molded.