Abstract: The dynamic polished rod load of beam pump in CBM wells is the important basis of designing and selecting correctly pumping units and sucker rod and determining motor power. Based on analyzing polished rod load of beam pump, the calculation model of dynamic polished rod load was established, and its variation and calculation for its extreme value were developed by solving the model. The results show that it is necessary to consider the inertia load and vibration load while calculating polished rod load. Dynamic polished rod load plays a major part in polished rod load. Following pumping water, the part decreases rapidly and it becomes small after entering stable production. The dynamic deformation is also relatively small. It is the first time to calculate more precisely polished rod load of beam pump in CBM wells by this method, which provides reasonable basis for design and calculation of beam pump.