The invention relates to a modulation strategy for suppressing non-isolated photovoltaic system leakage current. The technical characteristics are that the modulation strategy comprises the following steps that an H-bridge cascaded photovoltaic system is divided into a first-half period working mode and a second-half period working mode. Inverter output voltage is 0, -Vpv and -2Vpv under the first-half period working mode, the lower level switch state is maintained to be unchanged, a switch S21 and a switch S24 are turned off, and a switch S23 and a switch S22 are turned on. Inverter output voltage is 0, +Vpv and +2Vpv under the second-half period working mode, the upper level switch state is maintained to be unchanged, a switch S11 and a switch S14 are turned on, and a switch S13 and a switch S12 are turned off. An H-bridge cascaded multilevel inverter acts as the base, and half of the number of the required carriers is reduced in comparison with that of a conventional modulation strategy starting from the modulation strategy without changing the system structure or increasing system cost so that calculation amount is greatly reduced. Besides, common-mode voltage is controlled within a certain range so that an effect of suppressing leakage current is achieved.