Abstract: Carbonic oxide(CO) can effectively reduce the risk of hydrogen embrittlement(HE) of hydrogen pipelines by inhibiting the adsorption of hydrogen molecules on the inner wall, which provides a potential solution to ensure the safe hydrogen transportation by the in-service pipelines. However, the influence of CO on HE index is related to the microstructure of steel materials. A case study was carried out to X80 steel, for which slow strain rate tension tests were conducted on the base material, pre-strained base material and two different types of root weld under the conditions of nitrogen, hydrogen and hydrogen + 0.1% CO(by partial pressure, the same below), so as to study the HE index and CO influence degree of pipe body in different states. According to the results, the HE indexes of the base material, pre-strained base material, automatic weld and manual weld under the condition of 12 MPa pure hydrogen are 35.75%, 43.72%, 39.28% and 33.66%, respectively, showing a very high risk of hydrogen embrittlement. However, the hydrogen embrittlement sensitivity could be reduced greatly with the addition of CO. Specifically, adding 0.1% CO can lower the HE index of the four kinds of specimens to 15.66%, 17.54%, 23.16% and 25.33%, respectively. Among them, the HE index of manual weld has the minimum reduction, which is least affected by CO among the studying objects. Therefore, it is recommended to study the minimum CO addition that could inhibit the HE at various hydrogen partial pressures for the manual weld area in future.