A compact transmissive plasmonic waveguide mode converter which aims for the elimination of reflection and transmission of unconverted mode is proposed. The proposed scheme exploits a cavity formed by mode selective mirrors, which only allows two output modes: the transmission of the target mode and the reflection of the input mode. By appropriately tuning cavity lengths, the reflection of the input mode can also be suppressed to near zero by destructive interference, thereby all the residual outgoing modes are suppressed. The proposed device might be useful in the design of integrated photonic system since it relaxes the problem of unwanted reflection.