In this presentation, the universal structure of one-dimensional photonic crystal (1-D PC) is constructed, and its optical transmission properties are analyzed by transfer matrix method (TMM). A case that there are two kinds of medium as a period is studied in detail. It is concluded that the reflectivity in photonic band-gap (PBG) increases with the increasing of periodical number, and the bandwidth of PBG has direct relation with the difference between two kinds of dielectric constant, three methods for extending PBG are discussed. When defect layer is inserted, a defect mode appears in the PBG. The concept of optimal periodical number is presented, and it is found that this optimal periodical number is only relative to the ratio of dielectric constant (K). Using multi-objective optimization method, we educe the curve and equation relation between optimal periodical number and K for the first time. In addition, the change in the number of defect mode with the variation of the defect layer's thickness is analyzed, and it is explained by the theory of F-P cavity.