The successive commissioning of UHVDC transmission systems leads to changes in the operating characteristics of power system, the weak link in AC/DC hybrid power grid will change. In order to identify the critical lines in AC/DC hybrid power grid and avoid the occurrence of blackouts, a critical lines identification method based on steady-state security region for AC/DC hybrid power grid is proposed. The steady-state security region analysis method is used to construct the steady-state security region of AC/DC hybrid power system. Considering the power flow limit of AC line and the blocking criterion of DC inverter, the steady-state security margin calculation method for AC/DC hybrid power system is proposed combined with the branch flow model. The critical lines identification process of AC/DC hybrid power system is given with the steady-state security margin as an index. Finally, the simulation analysis is carried out on the actual power system of Jiangxi Power Grid, and the results demonstrate the effectiveness and rationality of the proposed method. This work is supported by National Natural Science Foundation of China (No. 51577075).