Based on the decoy state measurement-device-independent quantum key distribution (MDI-QKD) protocol, a MDI-QKD protocol using wavelength division multiplexing (WDM) technique is proposed. Both sides of communications, Alice and Bob, transmit multiple-channel signals with different wavelengths at the same time. The multi-channel signals are merged with optical multiplexer, and then a single fiber is adopted for long distance transmission. The third party, Charlie, separates the multi-channel synthetic signal with optical demultiplexer, and Bell-state measurement is performed for each sub-channel signal. Theoretical analysis results show that the key generation rate relates to the transmission distance, number of multiplexing and mean photon numbers. Numerical simulation results show that the proposed protocol greatly improves the system key generation rate. When the transmission distance is 150 km, the single-channel key generation rate is 0.17 Mbps. The key generation rates with 20 channels, 40 channels multiplexing are 3.34 Mbps, 6.68 Mbps respectively.