Abstract: In the project of Hami to Zhengzhou UHVDC system, to analyze the harmonic enlargement effect of UHVDC system on AC system, a method based on modulation theory is proposed in this paper. In this algorithm, the impedance calculation model of background harmonic caused by double twelve-pulse series UHVDC system in AC-DC system is deduced by modulation theory under normal operation mode. By this model the influence of the access of DC system to AC system on background harmonic amplification characteristics of original system is analyzed. On the other hand, Hami converter station will influence impedance of DC line during calculation process, and its calculation formula is deduced by using of a kind of average impedance method. In the end, the simulation model is built by using design parameters of Hami-Zhengzhou UHVDC DC system and the short-circuit impedance of Henan power system, which verifies the correctness of proposed method.