This paper studies the dynamic formulation of the string/rotor coupling system. The partial differential equation describing the lateral deflection of string, nonlinearly coupled with three ordinary differential equations describing the whirling of rotor vibration, are derived by means of the calculus of variation and Hamilton's principle. The main features in the formulation are (ⅰ) the mass and inertia of rotor are time-varying when the string is wind up or down, (ⅱ) the string length is time dependent, because the rotor has whring oscillation, (ⅲ) the contact point between the string and rotor is considered as the moving boundary since its unknown position has to be determined as part of the solutions.