The advantage of the overmoded slow-wave structures (SWS) in enhancing power capacity is analyzed theoretically. The dispersive characteristics of the rectangular corrugated overmoded SWS with the infinite length and the finite length are investigated through the theoretical method and the finite-element-method, respectively. The TM01 mode dispersive characteristics in the two cases are compared, and the results indicate that there are differences in the values of the longitudinal wavelength and its relevant frequency. The results can provide valuable reference to the design of the high-frequency structure of the high-power microwave devices.