A modulation method for a cascaded multi-level inverter system, applied by a controller for the cascaded multi-level inverter system, the cascaded multi-level inverter system comprising a plurality of inverter units, the plurality of inverter units being connected with the controller, the plurality of inverter units being connected with a plurality of DC sources respectively, the modulation method comprising: performing a maximum power point tracking control based on a voltage signal and a current signal of each of the plurality of DC sources and a voltage signal and a current signal of a power grid obtained by sampling, calculating a first modulation signal for suppressing power imbalance, and outputting the first modulation signal to each of the plurality of inverter units; calculating a reactive compensation current component based on the voltage signal and the current signal of each of the plurality of DC sources and the voltage signal of the power grid obtained by sampling; calculating a reactive current instruction value which is equal in size and opposite in direction to the reactive compensation current component based on the reactive compensation current component, characterized in that: the cascaded multi-level inverter system further comprises a reactive compensation device connected with the power grid, the plurality of inverter units are electrically connected in series, the reactive compensation device is provided on the AC side of, and in parallel to, the serially connected inverter units, and the modulation method further comprises: calculating an active current instruction value based on a DC-side voltage set value signal of the reactive compensation device, and a DC-side voltage signal of the reactive compensation device obtained by sampling; and calculating, based on the reactive current instruction value, the active current instruction value, and a current signal of the reactive compensation device obtained by sampling, a second modulation signal that, when provided to the reactive compensation device during operation of the cascaded multi-level inverter system, causes an output power factor of the cascaded multi-level inverter system to be 1 performing a maximum power point tracking control based on a voltage signal and a current signal of each of the plurality of DC sources and a voltage signal and a current signal of a power grid obtained by sampling, calculating a first modulation signal for suppressing power imbalance, and outputting the first modulation signal to each of the plurality of inverter units; calculating a reactive compensation current component based on the voltage signal and the current signal of each of the plurality of DC sources and the voltage signal of the power grid obtained by sampling; calculating a reactive current instruction value which is equal in size and opposite in direction to the reactive compensation current component based on the reactive compensation current component, characterized in that: the cascaded multi-level inverter system further comprises a reactive compensation device connected with the power grid, the plurality of inverter units are electrically connected in series, the reactive compensation device is provided on the AC side of, and in parallel to, the serially connected inverter units, and the modulation method further comprises: calculating an active current instruction value based on a DC-side voltage set value signal of the reactive compensation device, and a DC-side voltage signal of the reactive compensation device obtained by sampling; and calculating, based on the reactive current instruction value, the active current instruction value, and a current signal of the reactive compensation device obtained by sampling, a second modulation signal that, when provided to the reactive compensation device during operation of the cascaded multi-level inverter system, causes an output power factor of the cascaded multi-level inverter system to be 1 the cascaded multi-level inverter system further comprises a reactive compensation device connected with the power grid, the plurality of inverter units are electrically connected in series, the reactive compensation device is provided on the AC side of, and in parallel to, the serially connected inverter units, and the modulation method further comprises: calculating an active current instruction value based on a DC-side voltage set value signal of the reactive compensation device, and a DC-side voltage signal of the reactive compensation device obtained by sampling; and calculating, based on the reactive current instruction value, the active current instruction value, and a current signal of the reactive compensation device obtained by sampling, a second modulation signal that, when provided to the reactive compensation device during operation of the cascaded multi-level inverter system, causes an output power factor of the cascaded multi-level inverter system to be 1 calculating an active current instruction value based on a DC-side voltage set value signal of the reactive compensation device, and a DC-side voltage signal of the reactive compensation device obtained by sampling; and calculating, based on the reactive current instruction value, the active current instruction value, and a current signal of the reactive compensation device obtained by sampling, a second modulation signal that, when provided to the reactive compensation device during operation of the cascaded multi-level inverter system, causes an output power factor of the cascaded multi-level inverter system to be 1