We present a new periodic all-metal slow wave structure, a coaxialstep-disc-loaded system and the dispersion characteristicsof the structure. By using the field-matching method, the dispersionequation and the coupling impedance of this structure are obtained. Thecoaxial structure makes the bandwidth broader than that of thenon-coaxial one. Compared with the coaxial disc-loaded andridged-disc-loaded structures, the pass-band of coaxialstep-disc-loaded structure is the broadest. The calculation resultsshow that increasing the step width and decreasing the step thicknesscan improve the bandwidth.