Terahertz integrated devices have attracted great attention due to their potential applications in communication systems. Although some functional devices based on terahertz spoof surface plasmon polariton (SPP) waveguides have been studied, multimode waveguides and multimode interference mechanism have rarely been explored. This work investigates several key functional plasmonic devices based on multi-SPP mode interference in the terahertz regime. The feasibility of multimode transmission on terahertz plasmonic waveguides and control of the propagation modes by varying the waveguide parameters are first investigated. Then, the interference mechanism between two or more propagation modes and the self-imaging principle are applied to plasmonic waveguides. Finally, a series of terahertz plasmonic functional devices such as a coupler, wavelength diplexers, and cascaded devices are proposed and simulated by exploring different interference lengths, frequencies, and cascades via the self-imaging principle. These devices have good performance in transmission response, crosstalk, bandwidth, and footprint, and are expected to play an important role in the development of terahertz on-chip communication systems.