The invention discloses a fault-tolerant control method and a fault-tolerant control system for a back-to-back converter of a wind power generation system. The fault-tolerant control method comprisesthe steps of: determining a faulty bridge arm among six bridge arms of the back-to-back converter, reconstructing the rest five healthy bridge arms into a five-bridge-arm converter, connecting corresponding phases of the original faulty bridge arm to one healthy bridge arm by means of a bidirectional thyristor, and defining the healthy bridge arm as a common bridge arm; detecting a current of thecommon bridge arm so as to avoid overcurrent of the common bridge arm under fault-tolerant control, and adjusting a rotation speed of a generator if the current of the common bridge arm is greater than a current early-warning value, thereby controlling a phase difference between corresponding phases at the generator side and the grid side on the common bridge arm to be within 120-240 degrees, andrealizing normal operation of a wind power generation grid-connected system. The fault-tolerant control method can realize the uninterrupted fault-tolerant operation of the system under the fault of the converter, can keep the common bridge arm to be non-overcurrent, and has good robustness.