Porous silicon carbide ceramics were prepared by coat mix process using silicon powders and phenolic resin as silicon and carbon resource. The formation mechanism of silicon carbide was proposed based on the liquid silicon infiltration mechanism, reaction dynamics and thermodynamics analysis. Results show that the formation of silicon carbide by the coat mix process includes the following phases. 1) Infiltration of liquid silicon into porous carbon gap. 2) Formation of silicon carbide particles through the contact and reaction between liquid silicon and silicon surface. 3) Fracture and falling off of silicon carbide layer from the carbon surface. 4) Formation of new silicon carbide layer and particles. 5) The repetition of phase 3) and phase 4) till the reaction is complete and the porous silicon carbide ceramics are formed.