The ridge-loaded coaxial radial line slow wave structure is presented in this paper. The dispersion characteristic and coupling impedance of this slow wave structure are investigated by electromagnetic simulator High Frequency Structure Simulator(HFSS). It is analyzed that the change of structure parameter affects the high-frequency characteristics. The results show that the inner conductor radius and the period length have obvious effect on phase velocity. Along with the decrease of inner conductor radius and period length, the phase velocity and operating voltage can be favorably decreased. In addition, the width of the ridge makes the coupling impedance increase, when this structure is used as the slow wave structure, the gain and efficiency of the traveling wave tube can be improved. Meanwhile the length of ridge has no obvious effect on the high-frequency characteristics. Because the ridge-loaded coaxial radial line slow wave structure is a new all-metal structure with the advantage of broad bandwidth, high power capability and lower losses, it has a large potential of application in millimeter traveling-wave tubes.