A reconfigurable waveguide antenna array design for the MIMO beamforming system is demonstrated in this article, which is fed by the multilayer substrate integrated coaxial line (SICL) feeding network. According to the design purposes, the reconfigurable waveguide transitions and waveguide linearized-polarized/circular-polarized antenna arrays have high flexibility to meet different requirements in beamforming applications. The multilayer SICL feeding network could achieve wideband performance for both the MIMO beamforming transmission and receiving system for K/Ka-band. Both simulated results and some preliminary experimental results are presented for the demonstration.