Toroidal magnetic field should match frequency of the electron cyclotron wave in pre-ionization process of the Tokamak device. The existing electron cyclotron wave frequency of the NCST (Nanchang Spherical Tokamak) is low. A new toroidal coil current generation scheme is proposed to match the current electron cyclotron wave frequency. A low current step is added before the existing flat top current of original scheme. After reviewing the original scheme of the toroidal coil power supply of NCST, two schemes of full control and semi-control are designed. Advantages and disadvantages of the two schemes are compared from four aspects: high order harmonics of voltage and current, controllability and ripple of current, modification cost and installation convenience, and finally the semi-control transformation scheme is selected. The power supply cabinet is modified according to the field conditions with minimum alteration. Actual test results show that the two current steps are connected normally, and the width and amplitude of low current steps are adjustable, which meets the transformation requirements.