Aimed at the properties of high power density, large distensibility, strong and burst force of working fluid in the high pressure air ejection launch system, the real gas effects were introduced, and the influence on the pneumatic ejection performance by structural parameters was analyzed. The physics parameters of specific residual thermodynamic energy and specific residual enthalpy of the high-pressure air were presented under the conditions of real gas, pneumatic catapult closed equations were established on the basis of real gas effects, and the numerical solutions were performed. By comparative study of the relationship between a variety of structural parameters and the ejection launch system performance，the change law for influence of structural parameters on ejection launch system performance and parameter selection law were obtained. The numerical analysis shows that the greater the cross-sectional diameter of flow opening, the larger the volume of the high pressure chamber is. The smaller the volume of the low pressure chamber, the ejection performance of the system is better, and results of dynamic performance analysis supply reference basis for the global fuzzy design of system. 