Abstract: Abstract: As a kind of advanced water-saving irrigation technology, sprinkler irrigation system is adaptable to many crops, soils, and topographic conditions. At present, the most representative and most widely used sprinkler of the high pressure sprinklers are vertical impact drive sprinkler. In order to study the arm movement law of the sprinkler and the influence of counterweight on the hydraulic performance of the sprinkler, with the vertical impact sprinkler as the research object, this paper analyzed the arm movement characteristics of the vertical impact sprinkler by use of theoretic and experimental methods. The arm movement model of the vertical impact sprinkler was established, and according to this model, the free and non-free arm movement times were deduced and then the calculation formula of arm movement period was obtained. In this calculation formula, the factor of counterweight position change was taken into account. The experimental study on the arm movement of Nelson SR100 sprinkler was made with high-speed photography, then the experimental results of arm movement period were compared with the theoretical values, and the effect of arm movement period change on the hydraulic performance of the sprinkler was studied by changing the counterweight installation positions. The results showed that: the free and non-free arm movement times both decreased with the increase of working pressure under the same counterweight installation position. The free arm movement time was considerably larger than the non-free arm movement time, and over 90% of one arm movement period was free movement. In the free movement period, the curve of the arm angular displacement approximated to a smooth semi-period sine curve with the change of time. The arm movement period values obtained through the experiment of high-speed photography well coincided with the calculated values, with most of the relative errors within 10%, which verified the accuracy of the theoretical period formula. In view of the calculation errors of the theoretical period formula, the theoretical period formula was corrected according to the experimental results, thus further improving the calculation accuracy of the theoretical period formula with all the relative errors within 3%. The maximum rotation angle of the arm increased with the increase of the distance between counterweight and arm rotational axis, which increased the arm movement period and decreased the frequency of breaking water jet. Under the same working pressure, the non-free arm movement time increased while the free arm movement time decreased with the decrease of the distance between counterweight and arm rotational axis. Within one arm movement period, the arm angular velocity approximated to a smooth semi-period sine curve with the change of time. The arm angular velocity and movement period were both related to the counterweight installation positions. The farther the counterweight installation position was from the arm rotational axis, the bigger the arm angular velocity and period values became. Both the increase of the sprinkler working pressure and the distance between counterweight and arm rotational axis could decrease the movement period of the sprinkler. When the distance between counterweight and arm rotational axis was short, the sprinkling irrigation intensity near the sprinkler obviously increased and the water distribution increased by about 40% within 3-10 meters; when the distance between counterweight and arm rotational axis was long, the sprinkling irrigation intensity near the sprinkler decreased correspondingly and the sprinkling irrigation intensity at the end of sprinkling irrigation increased correspondingly. The results can provide valuble information for the establishment of the arm design method of the vertical impact sprinkler as well as the reasonable adjustment of the arm counterweight installation position during the operational process.