Mn55Al45 alloy was prepared by the combined use of arc melting, high energy ball milling and tempering heat treatment process. The phase structure and the microstructure of the alloy powders were analyzed by X-ray diffraction(XRD) and scanning electron microscopy(SEM), then their microwave absorbing properties were analyzed by vector network analyzer. The results show that an increase of phase for Al2Mn3 and Al11Mn14 are accompanied by the increased milling time; the resonant frequency and absorption peak of ε″ and μ″ shifts towards lower frequency region simultaneously. A better result of electromagnetic properties was brought to the alloy powders when the ball milling time was set to 18 hours in first attempt. While the ball milling time was increased to 24 hours during later experiment, the minimum reflectivity value reached to -28 dB at a frequency of 16 GHz. The Al2Mn3 phase increase with an artificial increases of tempering temperature. The Mn55Al45 alloy dealt with 18 h ball milling time under 400 ℃ heat treatment exhibits preferably comprehensive microwave absorbing properties in the range of 6-18 GHz. The minimum reflectivity value and absorption peak frequency with the coating thickness of 2.0 mm, are -26.4 dB, 17 GHz respectively.