Abstract: Based on the theory of mangetohydrodynamics, the arc-extinguishing characteristics of C₄F₇N/CO₂ mixed gas circuit breaker and SF₆ circuit breaker are simulated and computed under 40 kA short-circuit current, and the thermal properties of two gases and the gas pressure in arc-extinguishing chamber are analyzed. The results show that the gas pressure generated by 9%C₄F₇N/91%CO₂ gas mixture during arcing process is smaller than that generated by SF₆ gas under the same air pressure, and the arc-extinguishing ability of 9%C₄F₇N/91%CO₂ gas mixture during arcing process of mixed gas circuit breaker is weaker than that of SF₆ gas. And the radial temperature of SF₆ circuit breaker is higher than that of 9%C₄F₇N/91%CO₂ mixed gas circuit breaker, and the arc radius generated during arcing process of 9%C₄F₇N/91%CO₂ gas mixture is larger than that of SF₆ gas. When the gas charging pressure of gas mixture is increased to 0.8 MPa, the temperature gradient is improved, the arc radius is reduced, which can efficiently enhance the radial cooling effect, accelerate the extinguishing of arc, and ultimately boost the arc-extinguishing ability of circuit breaker.