Application of flexible reducer such as parachutes in the development of projectile such as terminal sensing ammunition is briefly introduced. It is mainly based on the development practice of the terminal sensing ammunition parachute and the design requirement of the ammunition ordinary parachute, the concepts of parachute crown and structure coefficient of the crown load are proposed and defined, a general engineering calculation method for the strength calculation and checking of the crown was established systematically, in which the crown load and the crown bearing capacity were taken as the main parameters. The engineering practice shows that this method is suitable for the crown structure and strength design of ordinary parachutes with multiple types.