To solve the problems of low inhibition of parasitic mode and measurement accuracy is susceptible to interference of parasitic modes in traditional high power microwave online measurement devices, a high-power TM₀₁ mode coupling device is studied, which is applied to the Ku-band relativistic backward wave oscillator (RBWO) online measurement system. Due to the RBWO emission’s angular inhomogeneity, asymmetric modes is generated, and the traditional single-arm porous circular waveguide coupler cannot solve the competition problem between the TM₀₁ mode and the asymmetric modes, which generally leads to the detection waveform distortion and coupling degree judgment deviation and seriously affects the accuracy of the online evaluation of the output power of TM₀₁ mode of the RBWO. For this reason, combining the four-arm porous coupling structure with the TM₀₁ mode selection network, a novel online mode-selective coupling device is proposed in this paper. Using the field structure difference of different waveguide modes, the proposed device realizes the differential coupling of TM₀₁ mode and other parasitic modes and solves the problem of inaccurate online test power caused by parasitic mode interference. The simulation results show that the coupling degree of the proposed new coupler to TM₀₁ mode is more than 20 dB higher than that of the other modes, and the on-line test waveform and power measured in the high-power experiment are in good agreement with the radiated field test waveform and power, and the coupling stability is significantly improved.