This paper proposes a new modeling method for buck converter. The new modeling method, named bi-switching status method (BSM), for buck converter is obtained via the propositional calculus and linear constraint. Based on the mixed logical dynamical (MLD) model, BSM model of buck converter is obtained. Compared with other modeling methods, such as MLD method and state-space averaging method, streamlined modeling procedure is proposed which can be represented as a general framework. By using the BSM modeling method, the transition conditions between different operation statuses can be simplified. Moreover, the framework of BSM model has only one inequality constraint which can be easily analyzed in the entire work process. A numerical simulation of the BSM model for buck converter shows the effectiveness of the proposed method. And the methodology can be applied to other converter topologies.