Abstract: A mathematical model of a axisymmetric convergent-divengent nozzle isset up by multivariant linear regression analysis method. The relationships of pressure ratio at nozzle exit with shock to four geometric parameters, i. e. area ratio, half-convergent angle, half-divergent angle and radius ratio of throat curvature to throat, are considered. The pressure ratio only depends on the area ratio. Other parameters have little effect on it. The results provide the basis for testing and transonic calculation of the axsiymmetric convergent-divergent nozzle.