This paper introduces eigen-beamforming technique into the STBC multiplexing scheme. STBC multiplexing scheme integrates orthogonal space time block codes into spatial multilayers such as V-BLAST structure. This hybrid system can balance the spatial multiplexing and diversity gain efficiently. When perfect channel state information (CSI) is available at the transmitter, eigen-beamforming can be employed in STBC multiplexing system for the sake of minimizing error probability or maximizing mutual information. The proposed precoding hybrid scheme provides extra precoding gain comparing with the non-precoded one. Simulation results display that the new precoding STBC multiplexing system can improve the BER performance to some extent especially in lower dimensional modulation.