Horizontal well drill string is simplified into a horizontal rotating beam, the beam-bore-hole interaction model and the bit rock interaction model of horizontal well drilling string system are also analyzed. Based on the above analysis, the equations which describe the nonlinear dynamics of the mechanical system are derived by Timoshenko's beam model and extended Hamilton's principle. The system are analyzed in nonlinear dynamics based on the equations, the results show that: in the process of movement, horizontal beam is always in the lower right location of the wellbore; with the increase of weight on bit, radial amplitude of beam rises slightly, longitudinal amplitude of beam declines; with the increase of rotating speed, radial amplitude of beam rises, longitudinal amplitude of beam also rises. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.