This paper proposes a new method to predict an unstart of a supersonic wind tunnel. The pressure loss caused by both of a model and structures in the wind tunnel and cross-sectional area distribution are considered as well as blockage ratio in the conventional method. The prediction based on the proposed method was obtained by using quasi-1D numerical simulation. In order to validate the proposed method, a wind tunnel test was performed. The predicted maximum blockage ratios quite agreed with experimental ones except for a few cases in which strong shock wave-boundary layer interaction was observed. The method is expected to be available for various wind tunnels and it gives us more accurate prediction than that by conventional theory.