To improve test capability at intermittent blow-down supersonic wind tunnels, diffuser performance is important to keep the starting pressure ratio as low as possible. A parametric study on the tunnel geometry for the JAXA 1m × 1m supersonic wind tunnel (JSWT) was conducted by numerical simulation. It was found that the diffuser part with a shock train has large effects on the starting pressure ratio. It was also indicated that oblique shock waves generated from the model support strut and the second throat ramp contributed the efficient pressure recovery. To improve the diffuser performance, we propose a design methodology considering these shock phenomena and designed a modified geometry for the JSWT. Although the cross-sectional area distribution was only changed from the original one, its higher performance was confirmed numerically. This design concept was also validated experimentally with a 10% scale model tunnel at a Mach number of 3.0.