A new union structure for time-domain simulation of power system is presented， which is realized via double layer al-gorithm with build-in net-form chain. The inner layer is designed for implicit method whereas the outer one is designed for ex-plicit method. The explicit method can be used as a predictor for implicit method or as an independent algorithm. Net-form chain is a logical structure developed from orthogonal list technology for sparse storage. The interface between dynamic devices and their controls can be flexibly designed，so as the interface between dynamic devices and network. The blocking Jacobian ma-trix can be assembled expediently. The realization scheme for simulation software design using the new algorithm is given.