The utility model relates to a connecting structure of a flange and a tank. The outer surface of the metal flange is coated with a non-metal material layer. The portion between the flange and the tank is coated with a layer of sealant. The outer surface of a bolt rod is coated with a layer of sealant. The flange coated with the non-metal material layer is connected to the tank through a bolt. The outer surface of a bolt head, the inner surface of the tank and the inner surface of the flange are respectively coated with a sealing layer. According to the connecting structure, the flange is connected to the tank through the bolt, so that the problem that the metal flange and the tank made of a composite material are difficult to connect is solved, the connecting structure is suitable for the transportation tanks made of the composite material, the flange and the tank are connected firmly, and impact of liquid media and a sea valve in the process of transportation can be borne effectively. The connecting structure can further be applied to other conditions that materials of the flange and the tank are different.