Microwave absorption property of nano-Fe3O4 and its composite system were studied and absorption principle and the effect of composite component on microwave absorption property were analyzed. The results show that，in the Fe3O4/ BaTiO3 composite system, as the components are changed, the position of absorption apex are adjustable. Compare to the single material, the bandwidth of composite system is enlarged. When the mass ratio of Fe3O4 to BaTiO3 is 3:2 and the sample's thickness is 2 mm, the reflectivity of -10 dB bandwidth is 2.7 GHz; when the mass ratio of Fe3O4 to BaTiO3 is 2:3, the reflectivity of -10 dB bandwidth is 4 GHz. In the Fe3O4/PANI composites system, when the mass ratio of Fe3O4 in the composite system is about 35 %, the electrical conductivity of the microwave absorbency is the highest with its absorption maximum of -21 dB and the bandwidth of -10 dB, which is larger than 4 GHz.