The Network design theory has been developed for nearly twenty years, with the relative models and algorithms gradually becoming mature and reasonable. The network design theory and methodology are made use of to explore the model of the urban transit network. A method is presented for transfering a specific description of the urban transit network into a general description of the network. A research result of the urban transit network assignment is introduced to establish a new model of urban transit network optimization whose characteristics are discussed and decomponsed into a master problem and a subproblem that is useful to the further study of the model.