Abstract: By analyzing the relationship between the ambient temperature,radiation intensity and the operating temperature of the inverter components in the representative region,combined with the model study of the impact of the inverter life on the capacity ratio design,a calculation method for the failure probability of photovoltaic inverter components is established. On this basis,we study and analyze the comprehensive influence model of radiation intensity distribution,power generation over-allocation loss,inverter initial cost,inverter failure and reliability on capacity ratio. Based on the best comprehensive cost per kilowatt-hour in the life cycle of the photovoltaic system principles,the optimal ratio of photovoltaic modules and inverters was obtained. Taking Changzhou,as the case analysis of distributed photovoltaic systems,and considering the failure probability and lifespan of the inverter,it is found that the optional capacity ratio of Changzhou area is 1.44 which the comprehensive economy is the best. This conclusion provides a reference for the fine design of distributed photovoltaic systems.