In view of the application of jet piston ejection mechanism in vertical ejection system, the source item method and user defined function (UDF) technique of CFD fluent software are used to inject mass, momentum and energy of propellant gas into high pressure chamber, the jet flow field in the high pressure chamber and the low pressure chamber at the initial stage is analyzed numerically, the pressure and the velocity distribution and changes are obtained, and the influence on the ejection mechanism from the pressure and the velocity is analyzed. The computation results show that this method can well simulate the ejection mechanism characteristics of the inner flow field, and can provide a theoretical basis for ejection mechanism and propellant design.