The invention discloses a constant-magnetic-flux excitation control system suitable for a variable-speed generator and a constant-magnetic-flux excitation control method thereof. Conventional generators operate under the condition of fixed rotating speed and fixed frequency, and the adopted generator excitation control methods include voltage closed loop and current closed loop modes. When the generator rotating speed changes, overexcitation of electricity utilization equipment occurs due to the excitation modes. According to the constant-magnetic-flux excitation method, overexcitation of the electricity utilization equipment is ensured not to occur under the condition of any change of the generator rotating speed so that the special requirements for the excitation system in variable-speed operation of the generator can be met.