The invention discloses a positive and negative sequence coordination control system and method of a chain type STATCOM during an unbalanced voltage of a power grid. The system comprises a power gridsynchronous phase rapid open-loop detection module, a global direct current voltage control module, a positive sequence reactive compensation current calculating module, a positive sequence reactive instruction current calculating module, a positive sequence current decoupling control module, a negative sequence separating module, a negative sequence voltage calculating module, a power module average direct current voltage control module, and a voltage instruction synthesis and phase-shift carrier modulation module. According to the system and method, random disturbance of the voltage phase ofthe power grid is solved by employing the advanced power grid phase open-loop synchronous control method, the phase sequences of the output voltage of the chain type STATCOM device and the voltage ofthe power grid are separated in real time, the relation between the phase shift angle and the modulation ratio required by positive and negative sequence component coordination control is disclosed,and the positive and negative sequence component coordination control method of the chain type STATCOM device in the condition of the unbalanced voltage of the power grid is provided.