The invention relates to a method and a system for determining the capacitance value of a full-bridge sub-module of a DC circuit breaker. The capacitance value range of the full-bridge sub-module of the main flow branch is calculated according to parameters such as the maximum turnable peak current of a mechanical switch on the main flow branch. The maximum capacitance of the full bridge sub-module on the transfer branch is calculated according to the parameters such as the maximum time of capacitance transient voltage establishment after the transfer branch is locked. The minimum capacitanceof the full-bridge sub-module in the transfer branch is calculated according to the parameters such as the maximum current of the transfer branch at the time when the transfer branch is turned off. The range of values is then obtained based on the maximum and minimum values. The method is based on the operation mechanism of full-bridge DC circuit breaker and the selection of key equipment parameters. The calculation process is simple and the calculation results are reliable and accurate. It avoids the problem of solving dynamic circuit differential equations under multi-constraint conditions and can effectively meet the design requirements of full-bridge DC circuit breaker capacitance detailed parameters.