Abstract: Various content ratios of the hollow fly ash, ferrite and carbon black were used to analyze the main factors and the influencing mechanisms on absorbing properties of cement based composites. The results indicate that hollow fly ash is the main factor on absorbing property. The best absorbing bandwidth of less than -10 dB was achieved by the specimen comprised of 20wt% ferrite, 20wt% hollow fly ash and 10vol% carbon black as exhibiting 10 GHz, which is obviously better than none hollow fly ash samples. Improving material surface transmission performance and dielectric loss are the main effect mechanisms of fly ash.