The MnAl alloy powder was prepared by high-energy ball milling and heat treatment process. Effect of Al content on the microstructure and microwave absorbing properties of MnAl powder were investigated with the help of scanning electron microscope (SEM), X-ray diffraction (XRD) and network vector analyzer. The results show that with the increase of Al content, Al0.89Mn1.11, β-Mn and Al2Mn3 contents in MnAl alloy powder decrease while the Al8Mn5 content increases; as a result, the powder magnetic loss and dielectric loss increase, resonance frequency moves to the low frequency, and more absorption peaks appear. When absorbing layer thickness is 2.0 mm, the Mn40Al60 sample has a maximum absorption peak at 12.2 GHz frequency, the reflectivity minimum is –26.5 dB, showing a better bandwidth effect. MnAl powder absorbing mechanism of electromagnetic wave includes the electromagnetic loss in the inner of the absorber and the interference loss between the reflection waves from the back interface and the front interface