A simple polymer derivation method was applied in this paper to prepare a series of SiCN (MWCNTs) composite ceramics by adjusting the mass ratio of multi-walled carbon nanotubes (MWCNTs). The percolation threshold of the corresponding MWCNTs addition amount when SiCN(MWCNTs) composite ceramics exhibit the optimal electromagnetic wave (EWM) absorption performance was studied, the effect of different addition amounts of MWCNTs on reflection loss (RL), effective absorption bandwidth (EAB), electromagnetic parameters, impedance matching parameters (Z) and attenuation coefficient (α) of composite ceramics was analyzed, and the EMW absorption mechanism of the corresponding composite ceramics when MWCNTs addition amount fix at percolation threshold was discussed. The results showed that composite ceramics exhibited the best EMW absorption performance when the addition amount of MWCNTs reached the percolation threshold (10wt%): the minimum reflection loss (RLmin) was -37.9 dB, and the EAB was 2.8 GHz at a thickness of 2.4 mm; its RLmin was -21.7 dB, and the EAB reached 4.7 GHz at a thickness of 1.7 mm. By changing the sample thickness from 1.0 mm to 5.0 mm, the EAB containing the C, X and Ku bands can be acquired. Therefore, it is expected to be a promising candidate for the new generation of EMW absorbers due to its light weight, high efficiency and broad band.