At 818GHz，the microwave absorbing properties of graphite expanded and vitrified small ball mortars(GEVSBM) were tested by bow reflection method. Factors affecting the microwave absorbing property of GEVSBM, such as graphite content, EVSB use level, foaming agent use level and specimen thickness were studied. The results show that the microwave absorbing properties of GEVSBM are improved gradually with increasing graphite content, and the percolation threshold of graphite in GEVSBM is 10% of cement content. The reflectivity of GEVSBM is improved with the increase of use level of foaming agent or EVSB . The minimum reflectivity of GEVSBM is -20.68 dB when the graphite content and foaming agent use level are 10.0% and 2.5% of cement content, respectively, and the effective bandwidth less than -10.00 dB is 3.0 GHz. The reflectivity of GEVSBM is improved with the increase of material thickness, but the increase value amplitude of reflectivity reduces gradually. GEVSBM exhibits good performance in microwave absorbing and thermal insulation, which can meet the requirements of building electromagnetic radiation protection and building energy saving.