Based on studying the transform theory of the multiphase system and the rotor field oriented control (RFOC) of multiphase induction motor (IM), the mathematical modulation of RFOC used in multiphase IM with closed-loops of harmonic currents is modeled. It can be used to solve the problems of harmonic currents in the conventional vector control of multiphase IM. The fundamental current and each of harmonic currents can be controlled independently for decoupling. The harmonic currents can be restrained effectively. In the paper, a nine-phase IM is proposed to verify the hypothesis. Experimental results confirm the theoretical analysis well. The paper further achieves the technology of electronic pole changing (EPC) under RFOC of multiphase IM. It is feasible for multiphase IMto realize pole-changing without cutting off the power and the EPC proposed could effectively extend the operational range of constant power in multiphase IM.