To prevent electromagnetic wave pollution, the electromagnetic wave absorption capability and compressive strength of ultra-high performance concrete (UHPC) mixed carbon black (CB) and carbon fibre (CF) are methodically examined in K-band frequency range (i.e. 18–26.5 GHz). The mechanical properties, loss tangent, reflection loss, and microstructure of UHPC with the addition of CF and CB contents are determined via pressure tests, probe method, reflectivity tests, and scanning electron microscopy. The test results reveal that the compressive strength, dielectric loss tangent and reflection loss of UHPC are enhanced by the addition of CB and CF. When 0.5 vol.% CF is added, the reflection loss less than −10 dB in the K-band frequency range, and less than –15 dB in the frequency range of 21.68–23.38 GHz. By comparison, the reflection loss of UHPC with 0.5 vol.% CF is more than 1.8 times that of plain UHPC in the K-band frequency range.