Excellent electromagnetic shielding materials is the best choice to protect human health and maintain the normal operation of instruments and equipment. Silicon nitride-based ceramics containing carbon fibers, with high electrical conductivity and strong electromagnetic shielding performance, were prepared by using spark plasma sintering (SPS). Firstly, the introduction of carbon fiber led to an increase in the dielectric constant of the composite ceramics, the impedance mismatch of the sample surface was increased, so that more the electromagnetic waves were reflected. Secondly, since high content carbon fiber was introduced, a relatively complete three-dimensional conductive network was formed in the silicon nitride matrix, leading to a conductivity of 2.15 S·m^-1. The incident electromagnetic wave is absorbed due to the dielectric loss of the materials. Finally, when 7 wt.% carbon fiber was introduced, Vickers hardness, fracture toughness and electromagnetic shielding effectiveness of the composite ceramics reached 11.90 GPa, 6.05 MPa·m^1/2 and 46 dB, respectively.