The invention provides a device and method for treating municipal sewage by a plug-flow type vertical ternary cycling process. The device comprises a municipal domestic sewage tank, a plug-flow type vertical ternary cycling aeration tank and a vertical-flow type precipitation tank which are connected sequentially, wherein the water outlet of the municipal domestic sewage tank is communicated with the water inlet in the bottom of one side of the plug-flow type vertical ternary cycling aeration tank through a pipeline, and the water outlet in the top of the other side of the plug-flow type vertical ternary cycling aeration tank is communicated with the vertical-flow type precipitation tank through a pipeline; and the bottom of the vertical-flow type precipitation tank is communicated with the water inlet of the plug-flow type vertical ternary cycling aeration tank through a sludge reflux pump. According to the device and method, the volume of an aeration zone is reduced, so that, on the premise of guaranteeing the aerobic zone treatment efficiency, the volume of an anoxic zone is increased, the anoxic zone treatment capacity is improved, and the capacity of a reactor in denitrification and simultaneous nitrification-denitrification can be reinforced. The device and method are more suitable for massive-water sewage with low carbon/nitrogen ratio.