Abstract: Based on a drillstring dynamic model, whirl and other dynamic characteristics have been simulated by using the finite element method. Furthermore, a dynamic safety evaluation criterion has been established through calculations of the dynamic stress of drillstring. The dynamic safety factor for drillstring with stabilizers has been calculated. The results show that whirl motion is directly related to the dynamic safety of drillstring. Moreover, almost all dangerous sections are located below the neutral point. The dynamic stress of the drillstring can be reduced by adjusting rotary speed and stabilizers, thusly the dynamic safety factor of the drillstring will be increased. The dynamic safety factor of drillstring, which can represent its downhole motion state, is an important index of drillstring dynamic safety.