We have theoretically investigated surface magnetoplasmons (SMPs) in an yttrium-iron-garnet (YIG) sandwiched waveguide. The dispersion demonstrated that this waveguide can support topological unidirectional SMPs. Based on unidirectional SMPs, magnetically controllable multimode interference (MMI) is verified in both symmetric and asymmetric waveguides. Due to the coupling between the modes along two YIG-air interfaces, the asymmetric waveguide supports a unidirectional even mode within a single-mode frequency range. Moreover, these modes are topologically protected when a disorder is introduced. Utilizing robust unidirectional SMP MMI (USMMI), tunable splitters have been achieved. It has been demonstrated that mode conversion between different modes can be realized. These results provide many degrees of freedom to manipulate topological waves.