A basic idea of a quiet wind tunnel is described. A supersonic quiet wind tunnel(SQWT-120) is designed and built. SQWT-120 designed Mach number is 4.0, the nozzle outlet diameter is 120mm, Re is 0.46～1.78×10⁷and the operation duration is 6-60 seconds. The conclusions of calibrations demonstrate the actual flow Mach number is 3.8, ΔM/M is ≯±1.2% between x=160mm and x=438mm. The unsteady pressure disturbances are not larger than 0.1% before x=450mm, the operation time is 15s. The results show that the wind tunnel can be used to study the steady flow and unsteady flow.