To investigate the subsynchronous oscillation（SSO） problem of the steam turbine generators near the rectifier station of Guizhou-GuangdongⅡ±500 kV DC transmission link， the unit interaction factor（UIF） analysis and the time domain simulation methods are adopted. NETOMAC program is used and detailed system model suitable for SSO study is built for time domain transient simulations. Based on the analysis considering different operation modes and load flow situations， it is concluded that the unstable SSO can occur at islanded operation of rectifier station or if only a weak interconnection existing between rectifier station and the rest of the AC system. The design of a supplementary SSO damping controller of the HVDC transmission link is recommended.