In this paper, Quadrature Phase Shift Keying (QPSK) modulation and demodulation system based on a Substrate Integrated Waveguide (SIW) is designed and implemented for high-speed data transmission. The technique of orthogonal multiplexing allows two independent channels to transmit data simultaneously through one waveguide structure. For this proof-of-principle study, a fabricated SIW prototype with bandwidth of 14–28 GHz is designed and optimized through full-wave simulations. The QPSK system prototype is verified with different data rates and bit patterns. Experimental results demonstrate that the bit error rates (BERs) of the received data in channel I/Q are less than 10–12 and the transmission rate can reach up to 15 Gb/s for 27-1 Pseudo Random Binary Sequence (PRBS). The transmission data rate of the proposed QPSK system is two times higher than that of the conventional mixing system.