With the construction of 1100kV UHVDC transmission project in China, the DC bias risk of Xinjiang power grid under the common influence of multiple DC grounding electrodes has become a new problem to be solved by the power sector. Based on the complex network theory, a method of identifying the key sites of DC bias under the joint action of multiple grounding electrodes is proposed. The influence of DC bias between sites and between sites and lines under the joint action of multiple grounding electrodes is studied. The key index which has an important influence on the distribution of DC bias considering the uncertainty of influencing factors is proposed to measure the characteristics of the action of multiple grounding electrodes. And the calculation method is given. Taking the 750kV UHV AC system in Xinjiang as an example, the key stations that affect the distribution of bias DC under the joint action of ± 1100kV quasi-East UHV DC and ± 800kV Tianzhong DC are analyzed. Compared with the actual test data in October 2018, the results verify the feasibility of the method and the validity of the key station criteria. The research results in this paper can effectively identify the characteristics of a transformer substation which plays a fundamental role in the distribution of magnetic bias, and provide a useful reference for improving the early warning ability of DC magnetic bias of power grid transformer.