The paper presents a new multiplexing scheme, called convolutional multiplexing (CM), to achieve high diversity gain and high spectrum efficiency for OFDM-based systems. In this scheme, data symbols are spread onto several subcarriers by the convolutional spreader. Spectrum efficiency can be improved by two approaches: high order modulation and multi-code multiplexing. With the best spreading codes searched out, the multi-code multiplexing OFDM-CM system performs better in AWGN channel, but may lose diversity gain in frequency selective fading channels. On the other hand, the single-code spreading OFDM-CM system with high order modulation can achieve the maximum diversity order. Simulation results show that the multi-code convolutional multiplexing perform better than code-division multiplexing (CDM) for OFDM-based systems.