A Voltage Source Converter (VSC), as a grid connection interface for new energy generation, is widely used in power systems. A single VSC connected to an infinite grid via line impedance is built as a typical system. The output active power characteristics of the VSC under a weak grid condition are studied. On this basis, the damping and torque model of VSC system is established, and the damping coefficient directly determines the static stability of the system. The synchronizing coefficient indirectly affects the system stability by affecting the damping power phase. According to the phase relationship between the damping power and the synchronous power, the static stability of the VSC system is explained, and the influence of the parameters and operating conditions on the system stability is studied using the Bode diagram. It is indicated that the lower the current loop proportional gain, the narrower the PLL bandwidth. With unreasonable reactive power operation, the VSC system will face the risk of static instability. The grid-tied VSC system is built on Matlab/Simulink to verify the correctness of the theoretical analysis. This work is supported by the Natural Science Foundation of Inner Mongolia Autonomous Region (No. 2016BS0508) and Graduate Scientific Research Innovation of Inner Mongolia Autonomous Region (No. S20191142Z).