An input/output apparatus of a multiplexer is provided, including: a main tap and at least two branch taps of the main tap, where each of the at least two branch taps is configured to couple to a different resonant cavity in the multiplexer, and the at least two branch taps include a first branch tap and a second branch tap; a coupling polarity of the first branch tap is opposite to that of the second branch tap; and a coupling calculation frequency of the second branch tap is closest to a coupling calculation frequency of the first branch tap. The input/output apparatus of the multiplexer enables two channels with closest frequencies to use different coupling polarities. Because the coupling polarities are different, signals naturally do not interfere with each other, and signal interference between channels is eliminated in principle. The embodiments of the present disclosure further provide a corresponding multiplexer.