The Expanded time-frequency (ETF-) OFDM system uses the two-level spreading technology for data transmissions. This system has shown to be robust against the narrowband jamming in flat fading channels. However, in multipath fading channels, the orthogonality of its spreading codes can be destroyed. To mitigate the adverse effects of multipath fading, narrowband jamming, and white noise in ETF-OFDM systems, we propose a transmission approach that finds a proper spreading code assignment at the transmitter and adopts the zero-forcing equalizer at the receiver. Simulation results show that the proposed scheme enables the ETF-OFDM system to suppress the interference under multipath fading channels.