The relationship between circulating current and capacitor voltage unbalance is studied when single-phase NPC H-bridge cascaded inverters are connected in parallel. Through the equivalent circuit of circulating current, the mathematical relationship between the circulating current and the unbalanced capacitor voltage in a switching cycle is derived, and it is found that the difference of the unbalanced capacitor voltage is one of the excitation sources of the circulating current. Meanwhile, the circulating current will improve the capacitor voltage balance of a parallel inverter and worsen the capacitor voltage balance of another inverter. In order to establish a mathematical model of circulating current-capacitor voltage unbalance under periodic steady-state conditions, a modelling idea of “hypothesis-modeling-solving-checking-correction” is proposed to establish a circulation effect model based on blockage, which effectively avoids multiple factors. The complexity of modelling also improves the accuracy of the model. Simulation and experimental results show that the coefficient of determination for the fitting of the block-based circulation model can reach 0.963 8 and 0.945 0, which proves the accuracy of the circulation model.