According to the complicated unloading operation for drill rod and the poor clamping stability, new back-up wrench device for hydraulic top drive drilling rig was designed to adapt to the Continental Scientific Drilling. This device has the advantages of large clamping force, little damage to drill rod, stable performance and two-way floating, by which the frequent rod lifting is saved and the subsidiary operation time is greatly reduced by rapid makeup and breakout operation. And using ADAMS dynamics simulation software, the motion simulation analysis is made on the clamping mechanism to verify the rationality of the mechanism design.