Abstract: For a VVVF Inverter--fed vector control system of inductino motor, not only realized complet decoupling between the torque and flux curremt vector of induction motor, but also ensured the maxium and constant torque control during the dynamic case if it is regulated by current and speed double control loops. In this paper, the structure of current and speed double loops for SPWM inverter--fed induction motor's vector control system is investigated. For ensurring the flux orientation in case of motor's parameters being changed, flux control loop is realized through calculating magnetic energy of motor. The modulation frequency of SPWM converter is calculated through simulation of several main harmonic components.