Abstract: This paper analyses development process of a double nested extended nozzle by using the motion theory on center of mass,and a normal differential eguation of second-order is obtained.The axial displacement,velocity and acceleration of extended cones are obtained by solving the equation with Runge-Kutta's method of forth-order.This paper gives a sample of single nested extended nozzle.The computation results are fundamentally consistent with the results of test.The method in this paper can be used in the design of a nested extended nozzle.