Nowadays a number of HVDC power transmission projects are building in Sichuan power grid. Yet HVDC system will lead to subsynchronous oscillation (SSO) in the sending end of AC systems, and cause the consequent torsional oscillation of relative generators and shorten the life of shaft. So it is extremely urgent to assess the risk of SSO. And this is of great significance for the security and stability of power grid. In this paper, the Unit Interaction Factor (UIF), and Prony identification method which is used to extract oscillation eigenvalues and time domain simulation of electromagnetic transient are used to study the SSO characteristic of Sichuan power grid caused by HVDC. And these methods can be mutual authentications in the time domain and frequency domain. The study results indicate that in several individual mode of operation, the ±800 kV UHVDC system from Xiangjiaba to Shanghai may stimulate torsional oscillation of Luzhou thermal power generator unit more seriously. And the set of analysis methods can well assess the risk of SSO. It has a high application value that can be widely used in engineering.% 四川电网正在兴建多条高压直流(HVDC)输电工程，而HVDC可能引发送端交流系统次同步振荡(SSO)，造成相关汽轮机组的轴系扭振，缩短大轴寿命。因而评估电网发生SSO的风险迫在眉睫，这对保障电网安全稳定运行意义重大。提出将机组作用系数法(UIF)、Prony辨识算法提取振荡特征和电磁暂态时域仿真相结合，对四川电网由HVDC引起的SSO进行了研究，并在时域和频域两方面互为验证。分析结果表明，个别运行方式下，向家坝——上海±800 kV特高压直流输电系统可能引发泸州火电机组比较严重的轴系扭振情况，应予重视。这一整套分析方法能够很好地评估系统发生SSO的风险，有很高的工程应用价值，值得推广。
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