With the material of natural flake graphite and polyacrylonitrile-based carbon fiber, the compound interference material of expanded graphite and carbon fiber is developed using HClO3-H3PO4-CrO3 as intercalant and alcohol as dispersing agent. The morphology, conductive performance and millimeter-wave (MMW) attenuation performance are characterized by means of scanning electron microscope (SEM), four probe method and MMW dynamic testing systems. The results indicate that carbon fiber disperses uniformly among the expanded graphite as single fiber. Within certain mass range, the conductive performance of the expanded graphite is strengthened and MMW attenuation performance is improved obviously when mixed with carbon fiber. When the grain size of natural flake graphite is about 180 μm and the mass content of short carbon fiber with 1.5 to 2 mm length reaches 20%, MMW attenuation performance is the best.