The small disturbance signal model of three permanent magnet direct-drive synchronous generators (PMSGs) connecting to the grid via VSC-HVDC is established. By calculating the eigenvalues, it is found that there are close sub-synchronous oscillation modes in the system. When the wind speed and other parameters of the PMSGs change slightly, the eigenvalues of these close sub-synchronous oscillation modes do not change much, but the participation factors of these close modes change greatly, which indicates that the dominant factors affecting the oscillation are sensitive to parameter changes.