Voltage-source Converters (VSC) have significant interaction with the power grid after being connected to the weak synchronous grid, which easily leads to the stability problem of sub-synchronous frequency band. In order to study the mechanism and characteristics of this phenomenon, a small signal model of VSC grid connected converter containing PLL and voltage feedforward control loop is constructed. The eigenvalue analysis method is used to calculate the system oscillation modes, damping ratios, and related factors. The key factors affecting the stability and changing rules are analyzed. The results show that the system oscillation risk can be reduced by enhancing the grid structure and optimizing the phase lock loop and voltage forward feed filter parameters in the VSC converter. Finally, the analysis results are verified by electromagnetic transient simulation. This work is supported by Science and Technology Project of the Headquarter of State Grid Corporation of China (No. 52660019000A) and Fund of State Grid East Inner Mongolia Electric Power Company Limited “Study on the Risk Analysis and Defense Technology of the Subsynchronous Oscillation in Large-Scale Wind Power Transmission Grid” (No. SGMD0000DDJS1701168).