Multimode feed network is an important part of shortwave multimode multifeed antenna system. The design of feed network is a pivotal technique in the process of projecting antenna system. The substructure analyzing method of interconnect-net is based on the substructure matrix database which is formed by converting the net parameters into data. And it characterizes the non-uniform net units through the database directly. Then cascades the known database of the net units by means of ordinary net cascade method, thus performing optimization in the net. In the paper, the Substructure Analyzing Method of Interconnect-net is applied to designing multi-mode multi-feed network, a typical example of multi-mode multi-feed network optimization is given to illustrate the efficiency of the method.