The utility model relates to a blast cap with exhausting aid function, arranged at the upper end of the pipeline of the air exhausting duct extending out of the rooftop roofing and used in conjunction with the air exhausting duct. The blast cap comprises an upper and a lower plates, a connecting strut, the lower convex globe below the upper plate and the upper convex globe above the lower plate. The air outlet above the lower plate is directly butt jointed with the air exhausting duct extending out of the roofing and the direct butt joint mode of the air outlet and the air exhausting duct extending out of the roofing is that the cross-section area of the air outlet is greater than the area of the air exhausting duct pore and smaller than the circumcircle area of the air exhausting duct pore. The lower convex globe of the upper plate and the upper convex globe of the lower plate are rotary bodies with hyperbola as generatrix and rounded corners are arranged at the upper port of the air outlet of the upper convex globe of the lower plate. The upper and the lower convex globes are both on the central line of the blast cap, the four-edge external dimension of the upper plate is greater than that of the lower plate and the upper surface of the upper plate is a downward inclined plane from the middle to the periphery. The utility model has the advantages of simple structure, changing harm to benefit and not only preventing down-draught resulting from external wind interference but also having pumping action to the air exhausting duct.