Embodiments of the present invention disclose a transmission mode selecting method, an antenna TX/RX combination determining method, device, and system. The transmission mode selecting method includes: obtaining a first system capacity in a half-duplex transmission mode; obtaining a second system capacity in a full-duplex transmission mode, where the second system capacity is the maximum value among system capacities of a universal set of candidate antenna TX/RX combinations in the full-duplex transmission mode, or the maximum value among system capacities of a first subset of the candidate antenna TX/RX combinations in the full-duplex transmission mode, or a system capacity exceeding a system capacity threshold; and selecting a target transmission mode according to a comparison result between the first system capacity and the second system capacity, where the target transmission mode is a transmission mode corresponding to a larger value between the first system capacity and the second system capacity. That is, in the current channel condition where a multi-antenna system is running, a transmission mode enabling the system capacity to be increased or maximized is selected according to a system capacity increasing or maximizing rule, so as to achieve favorable or optimal system performance in current channel conditions of the system.