With the aim of dynamic simulation and control, a cell-level nonlinear state-space dynamic model of solid oxide fuel cell (SOFC) based on physical principles is built. It demonstrates that reactant diffusion processes from gas flow bulks to triple phase boundaries (tpb) play an important role in the dynamic behaviors of SOFC. The simulation shows that under certain condition, the effect of the double layer capacitance may be neglected. The phenomenon of slow rise of voltage in current interrupt experiment may be explained by the dynamic behaviors of partial pressures in the vicinity immediately above tpbs. A new equivalent circuit is proposed. The results are demonstrated through simulations.