Nanopowders of 3C-SiC were synthesized at 1400°C for 3 h in Ar atmosphere via catalytic carbothermal reduction reaction method with industrial diatomite powders and phenolic resin as raw materials and ferric nitrate as catalyst precursor. XRD, SEM and TEM analysis were employed to characterize the phase composition and microstructure of the final products. The effect of temperature, catalyst content and holding time on the formation of the SiC powders was investigated. The results show that: 1) 3C-SiC can be synthesized at 1400°C for 3 h with 1.0% (mass fraction) Fe as catalyst. In the contrast, for the case without Fe catalyst, only small amount of 3C-SiC was obtained in the final products under identical condition; 2) The as-prepared 3C-SiC nanopowders are granular in morphology, and the diameters of most particles are in nano-scales; 3) Density Functional Theory (DFT) calculation results further show that the Fe catalyst played important role in breaking the Si-O chemical bond.