The invention discloses an optimal fault-tolerant control method and system for a poly-phase motor based on online current optimization. The method, which aims at a speed regulation system composed ofa poly-phase motor with symmetrical windings, includes the following steps: constructing a reconstruction vector, using the reconstruction vector to redistribute a current command, and obtaining a current command value with minimum loss; carrying out Park transformation on the actual phase current of the motor to obtain a current feedback value; inputting the current feedback value and the current command value with minimum loss to a positive and negative sequence PI current controller to calculate a motor voltage command under the positive and negative sequence coordinates; carrying out reverse Park transformation on the motor voltage command to get a voltage command of each phase; and outputting the voltage command of each phase through a sine wave pulse width modulation method to complete optimal fault-tolerant control on the motor current. Through the method, a current command value with minimum loss under any open-phase fault and any load size in the maximum load range can be obtained through online calculation, and an excellent torque ripple suppression effect can be achieved.