The deployment rule of the single extendible exit cone is investigated in this paper. The cone is divided into four pieces in the direction of meridian. As a result

the actuators are just located in the middle of each piece. Each piece of the cone can thus be simplified into a moving mechanism. Based on the force balance between the mechanisms

a second ordes ordinary differential equation is derived by applying the mass center movement theorem. The numerical solutionof the equation can be obtained by means of the fourth order Runge-Kutte method. The time-dependent parameters

such as position

velocity and accelerationof the external sleeve of the actuator relative to the internal tube

and the time needed for fully deploying the cone can then be obtained by using other related equations.