The "supersonic biplane theory" is one of the most effective concepts to reduce the sonic boom in supersonic flight. However, the unstart characteristics, that is choking phenomena, was one of the critical issues that disturb the shock wave cancellation between the wings of supersonic biplane. In this study, the shock wave behavior around the wing was visualized and discussed by Schlieren method. And the flow characteristics on the wing were investigated by pressure sensitive paints (PSP) and CFD analysis under the design and offdesign Mach number conditions. These surface pressure profiles on the wing of supersonic biplane clarified that the wave interaction between the oblique shock waves from the leading edge and Mach waves from the wing-tip was dominant of the start/unstart characteristics. Moreover, the two aspect ratio models were used to investigate the effect of aspect ratio on the shock wave interaction and cancellation. The lower aspect ratio achieved the shock wave cancellation, but the higher one did not achieve it under off-design Mach number conditions. The three dimensional flow outward, mainstream Mach number, boundary layer growth on the wing, model dimension and paint thickness were some influencing parameters of start characteristics.