The invention relates to a forming device for a U-shaped lazy arm of a crane. The forming device comprises a first gantry frame and a second gantry frame which are parallelly arranged and further comprises a main beam arranged between the first gantry frame and the second gantry frame and used for connecting the first gantry frame with the second gantry frame, a lifting U-shaped upper mold is arranged on the main beam, and a U-shaped lower mold matched with the U-shaped upper mold to form the U-shaped lazy arm is arranged below the U-shaped upper mold. The U-shaped lower mold is arranged on arear bracket located on the same side of the first gantry frame and the second gantry frame, and a front bracket is arranged on the other side of the first gantry frame and the second gantry frame. Aplate feeding device horizontally moving in the length direction of the first gantry frame or the second gantry frame is arranged on the front bracket, a to-be-formed plate is completely pressed intothe U-shaped lower mold through the lifting U-shaped upper mold, one-time forming of the U-shaped lazy arm is completed, and the forming device is simple and reliable in structure, convenient to operate, and high in forming quality.