Abstract: The single-phase cascaded multilevel inverter（CMI）has attracted much attention in the research field of grid-connected photovoltaic systems in recent years because of its advantages such as the module-level maximum power point tracking（MPPT）,the module-level shutdown, the low harmonic content of multilevel output voltage and the structure integrating the optimizer and the grid-connected inverter. The single-phase transformerless photovoltaic cascaded multilevel inverter（PV-CMI）with the independent DC bus structure is taken as the research subject, and two existing problems are summarized. One is that the lack of isolation circuits may lead to leakage current problems, and the other is that the independent DC photovoltaic inputs between the units in single-phase PV-CMI may lead to power imbalance problems. These two problems threaten the safe and stable operation of the single-phase PV-CMI, thus greatly limiting the promotion and application of the single-phase PV-CMI. The basic principle and topology characteristics of the single-phase PV-CMI are analyzed, and the existing research schemes are summarized on the two key problems of its leakage current and photovoltaic power imbalance. Finally, the current technical challenges and the future development trends of the PV-CMI are presented, which provides references for the subsequent research.