For a single converter, AC and DC are coupled to each other; for a converter cascade system, the converters interact with each other in some cases, which in turn affects the stability of the system. In this paper, the HVDC system is taken as the research object, and the harmonic linearization method is adopted to consider the interaction between the DC side and the AC side to analyze the stability of the cascade system. Experimental tests are performed to verify the effectiveness of the proposed method. According to the results, relevant measures to improve the stability of the cascade system are proposed.