A ramjet with a center preburner was designed for the purpose of intensifying the ability of ignition and flamehold characteristic at low pressures. The numerical studies were conducted to investigate the spray and combustion characteristics in the simplified model combustion. The result reveals that, with the change of the work conditions, the convergent rear of the preburner has a different impact on the interaction between the inner and the outer flow. The flow at the entrance of the preburner will accelerate due to the increase in the pressure drop and combustion occurring downstream affects the flow characteristics in the preburner.