In deepwater dislling, 36-inch conductors are set using the drill string with mud motor. For tihs case， the torsional energy equation and kinetic energy equation of the drill string are established， and the natural frequency equation of the drill string under this condition is solved according to the Rayleigh principle of the energy cnservation equation. The frequency equation takes into account the effect of nine geometric parameters， i.e., the length， the outer diameter and the inner diameter of the drill string, those of the dirll collar， and those of the conductor. Such an equation has strong practicabibty and make the theoretical analysis fit better with the actual situation. Therefore， the use of this set of analysis and calculation method is of great significance to guide the resonable determination of engineering parameters， the avoid ance of torsional vibration， jeopardize the ensurance of down-hole operation safety and the pervention of down-hole accidents.