Abstract: The features of the space environment conditions faced by the large flanges for spacecraft interfaces are aimed.The great loss caused by the spacecraft leakage happened in the history of the aerospace is considered.The theoretical calculation of the strength and the stiffness of spacecraft flanges is carried out.Thc selection of thc flange and its optimizing design are discussed in order to assure the safety of sealing flanges designed for spacecraft.The calculating methed and the design proved to be available through the practice.