Abstract: Insulating flanges (insulated joints) are indispendable and important pipe fittings in CP system of pipeline. However, the problem exists, which the insulating flange can not meet the needs of the inslutaing property specified in standard during installation. For this reason, an analysis has been made on this problem, and it is pointed out that a bad quality in manufacture, insulating material and installation affects the insulating property. In order to ensure the insulating property, the paper puts forward suggestions: ① insulating flange (insulated joint)must be inspected on the insulating property in accordance with code concerned before use; ② giving a request of water absorption rate at static pressure to manufacturer; ③ a peep hole should be set up at the insulating joint in order to inspect and disposal timely the leakage from sealing ring or insulated gasket.