Abstract: The Ladder slow-wave structure is an important and useful slow-wave system, which is used in power and high-power traveling wave tube, and ladder slow-wave structure has many advantages, such as simple structure, low cost, high power capacity, and good heat dissipation and so on. At present, the research of ladder slow wave structure has been in progress. It has been analysed of the ladder slow-wave structure mode, and it has been analysed of the theories and process in the using of CST software eigen mode solver in calculation of dispersion characteristic. On this basis, the design of the ladder slow-wave structures at the operating point of 100GHz and 225GHz has been given, and it has been studied of the impact on the dispersion characteristics from the size of the ladder slow wave structure.