The invention relates to a screw cap for a tire, comprising a hexagon flange-shaped nut end head, a cylindrical sleeve coaxially connected with the nut end head and a gasket vertically sheathed between the nut end head and the sleeve. A semi-circular first lug boss and a semi-circular second lug boss are installed at intervals at the peripheral region of the sleeve at the connection position withthe nut end head; a concave portion is formed between the first lug boss and the second lug boss; the inner side at the upper end of the gasket is provided with a contact portion matched with an inclined plane formed between the concave portion and the first lug boss, and a semi-circular groove for containing the second lug boss; the second lug boss is matched with the groove through a spherical surface; the upper end part of the gasket and the lower end part of the nut end head are semi-spherical surfaces the shapes of which are matched with each other so that the gasket surface is vertically pressed with uniformed pressure, thereby ensuring the uniform stress of the gasket, prolonging the service life, and ensuring the use security.