The utility model provides a forming die for a car frame longitudinal beam. The forming die comprises a concave die body and a convex die body. A plate arranged between the concave die body and the convex die body is made to form a bent n-shaped car frame longitudinal beam structure through matching between the concave die body and the convex die body. The concave die body comprises a concave cavity corresponding to the car frame longitudinal beam in shape. The concave cavity comprises a first face and a first side face connected with the first face, wherein the first face is horizontal, and the first side face is vertical. The convex die body comprises a second face matched with the first face, wherein the second face comprises a first part parallel to the first face and a second part which is sunken relative to the first part. The convex die body further comprises a second side face matched with the first side face, wherein the second side face inclines relative to the first side face. The convex die body stretches into the concave cavity of the concave die body in an inserted mode. The second side face inclines away from the first side face from top to bottom. The forming die can solve the problem that the car frame longitudinal beam rebounds in the press forming process.