In hybrid renewable power systems, the use of a multiple-input DC-DC converter (MIC) leads to simpler circuit and lower cost, compared to the conventional use of several single-input converters. This paper focuses on the optimizing design of MICs. Different to single-input DC-DC converters, the parameters design of MIC is very complicated. This is because the converter will operate in multiple operation modes, in which the power sources can power the load either simultaneously or individually. Moreover, in order to achieve soft-switching of all the switches in the multiple operation modes, parameter design of soft-switching dual-input full-bridge DC-DC converter is proposed, which is considered the multiple operation modes simultaneously. The design method and a prototype is built to verify the theoretical analysis.