We present a dynamically reconfigurable spoof surface plasmon polariton (SSPP) waveguide capable of bidirectional switching between perfect absorption and perfect transmission through active control. Nonlinear varactor diodes are integrated into the waveguide, enabling degenerate phase matching between pump and signal waves via voltage-tuned dispersion engineering. Three-wave mixing processes are established, allowing bidirectional phase-controlled transitions from destructive to constructive interference. The proposed SSPP waveguide overcomes traditional pumping constraints with its bidirectional configuration, supporting both forward- and backward-propagating pump-signal configurations and permitting signal amplitude modulations at both the transmitter and receiver ends. Experimental characterization demonstrates remarkable signal gain tunability: the forward pumping configuration achieves a dynamic range spanning from −69.50 to +1.04 dB, while the backward configuration spans from −70.49 to +1.45 dB. This work provides new design paradigms for microwave coherent systems and advances the development of reconfigurable electromagnetic devices for adaptive energy harvesting and high-speed signal processing applications.