The variation of pressure oscillation amplitude in closed supersonic duct was studied with different flight Mach number or duct length-to-diameter ratio by unsteady numerical simulations. The results show that the pressure oscillation extreme value and amplitude are independent of duct length-to-diameter ratio. When flight Mach number increases, the pressure oscillation extreme value nearly linear increases with total pressure of supersonic flow. The absolute amplitude increases, but the amplitude tends to be no change relative to total pressure. In duct, the average oscillation pressure is close to total pressure after shock. The maximum pressure possibly exceeds total pressure of supersonic flow.