Abstract: The check ring connecting structure between composite case of solid propellant rocket motor and conic fixed body of a nozzle is of great advantage to decrease the mass and stress.In order to analyze this structure an equivalent orthotropic axi symmetric body is used to represent composite motor case and a FEM program for contact problem with large deformations is developed.The calculation shows that the relative displacement between check ring and nozzle conic fixed body and the angle between two sealing surfaces are very small under the working load.This fact indicates that the check ring connecting structure is safe in application.The relations of the concerned structure parameters to the deflection of the sealing surfaces are discussed.For the sake of comparison it is also found that the structure of submerged fitting increases the contact area magnificently and decrease the angle between two sealing surfaces to 4.02%.