Abstract: The O-type Cerenkov devices usually use a section of finite-length slow wave structure (SWS) as the core region of beam-wave interaction, the longitudinal resonances occur when the electromagnetic wave is reflected at the terminal of SWS and affect the beam-wave interaction of the device, and as a result investigating the high-frequency characteristics of finite-length slow-wave structure is important for the designing of O-type Cerenkov devices structure. Using the field-matching method, the expression for the reflection coefficient of electromagnetic wave reflected at the intersection face of straight waveguide and SWS was derived, and the relationships between the reflection coefficient and the longitudinal wave number and SWS terminal phase were acquired. Then using the transmission line theory and the calculation results of reflection coefficient, the longitudinal resonant condition in finite-length SWS was brought forward, and using this method we analyzed the longitudinal resonances in finite-length SWS. We calculated the longitudinal resonant frequencies of a section of finite-length SWS. Theoretical and simulation results were close to each other. The resonant frequencies of a type of nonuniform SWS cold cavity were simulated; the frequencies and field distribution of the resonant mode in nonuniform SWS were close to the relevant uniform SWS. Consequently the longitudinal resonant condition could also be used for the analysis of nonuniform SWS with finite length.