Dimensionless dispersion curve equation of one-dimensional periodic structure with three oscillators is deduced. It is proved by singularity theory that the topology of the dispersion curves does not change with mass and stiffness parameter. Then the equations of initial and final frequencies of the bands are presented. The method of band gap design is gained consequently. Two examples i.e., the system with the same stiffness and the system with the same mass, are given to verify it. The method may provide the reference for designing the band gap of the photonic and phononic crystals.