The invention relates to modulation method and implementation circuit for a unit vector to carrying out the time-delay superimposition of a multi-level space vectors, belonging to the field of electrical energy conversion and solving the problem that a cascading type inverter is modified by adopting a traditional multi-level SVM (Support Vector Machine) method, vector selection is needed, and a calculating method is complicated. The modification method comprises the following steps of: equalizing the unit of each stage of a three-phase N-stage unit cascading type inverter into two double-level three-phase inverters; calculating the output voltage vector of a left bridge arm equivalent inverter of the first-stage unit at the beginning time of a calculating period by adopting a double-levelspace vector modification method; acting the voltage vector on a left bridge arm equivalent inverter of an ith-stage unit after the voltage vector is delayed by (i-1)Ts/2N; and after delaying, actingon a right bridge arm equivalent inverter of the ith-stage unit so as to realize the output of a multi-level PWM voltage wave form. The implementation circuit consists of a DSP (Digital Signal Processor) chip and a field programmable gate array (FPGA). The invention is used for Modulating the multi-level space vector of the cascading type inverter.