Valves in self-blast circuit breakers are important parts for the design of arc extinguishing chamber. Aiming to investigate the influence of the valves, the valve motion models are proposed. The gas flow field and the arcing process are simulated with the method of magneto-hydro-dynamics. The average pressure and the force on both sides of the valves are compared. The results show that the closure of the non-return valve around the current peak accelerates the pressure rising in the expansion volume. The opening of the pressure relief valve reduces the overpressure in the compression volume. Under the calculation conditions used in this paper, the non-return valve motioned around 21ms and the pressure relief valve motioned around 24ms.