An E-band waveguide diplexer fabricated by precision machining with harmonic suppression is presented. The simulation result shows the insert loss in 71-76 GHz and 81-86 GHz pass-band is better than 1 dB. The isolation is greater than 60 dB. In the second harmonic pass-band 142-152 GHz and 162-172 GHz，the suppression is greater than 20 dB. The measured results show the insert loss is less than 0.7 dB in pass-band and the isolation is greater than 60 dB，the suppression of harmonic is 20 dB. Simulation and measurement results match very well. The excellent performance of the diplexer ensures that it is suitable for 5G E-band communication backhaul networks and has a good prospect of engineering applications.